There's a plastic rectum on the examination table - a rigid tube designed to show Cilla Black just what a state-of-the-art endoscope can do. A doctor unsheathes the instrument (plus handy-claw attachment) and manhandles a jelly baby lodged in imaginary rectal tissue. For demonstration purposes only, of course. In case he's leaving too much to the imagination, he refers to a vividly-illustrated book on Inflammatory Bowel Disease. “And that (pointing at his new full-colour tv monitor) is looking directly at the lining of the colon”. It's enough to test the mettle of any light entertainment personality, and Cilla, patron of Wycombe Hospital's endoscopy appeal these past two years, is looking a little peaky. “Really great”, she smiles. But she won't be partaking of the refreshment provided.

Cilla unveils the commemorative plaque without the help of the assembled NHS dignitaries. “I have got curtains at home, you know”. She's very polite to the important Misters and Sisters, but it's the people waiting in reception that she's really come to see. There's Mrs Scattergood, who knitted matinee jackets and baby cardigans to raise funds. And Wynne Tidbury, who could always be counted on to rattle a collecting tin. Last time they met, Cilla admired Wynne's leggings - so Wynne parcelled up and sent her a pair. When Cilla finally says t'ra, Gary drives her home. He's just put black windows in the Merc Limo, and Cilla can't help feeling a little embarrassed. “I feel self-conscious. I don't think I'm that kind of a star.” Like Evita, she doesn't like to be distanced from her public.

Cilla - who understands irony - insists that when the double gates close, electronically, on her 16-acre Buckinghamshire estate (with putting green), she's just an ordinary housewife. “I'm very true to my stars” she says. “I'm a Gemini, so I'm two different people. When I'm on, I'm on. Eyelashes, outfit and everything. But I get home and it's the old trouser suit. Or something you can just throw in the wash. When I'm at home I change into Mrs Willis.” Admittedly Mrs Willis doesn't do windows. Or dusting. Or ironing. That's down to Penny the housekeeper. But this morning Mrs Willis noticed that the back staircase needed doing, before breading two veal escalopes and preparing a shepherd's pie. All for the love of Bobby and the boys. Mum and Dad have only ever spent three nights apart - the nights their children were born.

Cilla likes the housekeeper to call her Mrs Willis. And it's Mrs Willis to all the teachers at school. “It started when the children were small. I remember Ben, my middle boy. One lad grabbed round my knees - he was that small - and said 'I love you, Cilla Black'. Ben said 'No, this is my Mummy - that's your mummy over there. You should go and love your Mummy'. So I decided it was wrong for the teachers to call me Cilla when Mrs Wilson was over there. Plus, I didn't want to come home and have my own kids calling me 'Cilla'. It's just basic respect.” Bobby has pet names for his loving wife. Most of them are unprintable. “If we meet on the stairs in the morning, he says...” “Shift!” says Bobby. “No. If we meet on the stairs in the morning, it's Cill (with a soft c).”

For the last 35 years Bobby has had to get used to being Mr Cilla. He's now Cilla's manager, but the young Bobby Willis was a singer in his own right. He even wrote Shy Of Love, the B-side on Cilla's first single. Now he has a recurring dream - that he has to go on stage to perform but can't remember the words to the song. Psychoanalysts would have a field day. “Initially” he says, “being a Liverpool man of my generation, it was difficult not being the major bread winner. I figured Cill's pop career would last six years maximum. I thought we'd work for ourselves - open a hairdressers, whatever. But she kept going. Then the kids arrived. All the pieces of the jigsaw fitted together, and I realised we were working toward the same goal.”

Bobby has been managing Cilla since 1967. He's the one who renogotiates an LWT contract worth an estimated £3million every two years. “They say I'm worth a total of £12million” says Cilla, “but I'm still looking under every bed in the house. It is silly money I suppose.” Bobby leans forward. “It isn't silly money”. “Bobby says it isn't silly money” says Cilla. But then it's Bobby's job to say it isn't silly money. Cilla's not bothered - the knowledge of being Britain's highest paid celebrity is enough. She doesn't carry cash. She doesn't understand the paying in/withdrawal system on her new Sainsbury's bank account. She's never even written a cheque. But Bobby is different. Bobby accounts for everything.

Cilla will do the lottery if she remembers. Bobby will only do it (“you've still got more chance of being run over”) if it's roll-over week. “If it's £20million, I'll have a go. I'll spare a pound for that. But I'm not going to be bothered with the odd million.” This year the pair went to Ascot. “Some friends had a box” says Cilla. “Actually, they own it” says Bobby. “I asked”. “Anyway” says Cilla “we set a ceiling on losses of £200. That's a hell of a lot of money to throw away like that - for a woman who still saves butter wrappers to grease her cake tins.” Cilla insists she's the same woman she always was - even if these days it's not butter, it's low-fat spread. That's why Bobby won't get the Ferrari he's after. He'll have to make do with the Roller.

Cilla even calls the camera 'Bobby'. There's an old showbiz story that, to improve her conversational tone, she imagines she's talking to her husband. “Rubbish. I'd never get a show done if I imagined the camera was him. I know he'd be criticising every little mistake. I like him in the building - if there's anything wrong, he'll be down on the studio floor in a minute - but that's it.” He watches her from a monitor, back in the dressing room. It means he can cry in peace when she's filming ITV's three-hanky reunion show, Surprise, Surprise. “It always gets me” says Bobby. “It's not just me. These SAS guys said 'That bloody programme - we just fill up'. We used an Australian crew for the special Cilla did down under. The cameras were wobbling. All these big butch ockers - crying. I go all the time.”

No tears tonight - Cilla's filming Blind Date. But Bobby's still not happy. This time it's down to the creases in her frock. Part one, and last week's couple are back from a weekend in Jersey. He was too insistent. She wasn't. “He kept coming too quickly” says the young girl. A few titters. “Would you handle it differently next time?” asks Cilla. The audience erupt. Cilla looks to camera and says “That is definitely an edit”. For a moment, Cilla looks prudish. “I do understand innuendo. But not always at the time. I don't think 'the other way'. If you do that leery look to camera it sets things back on the old road (a reference to 1994 when Blind Date was sanctioned by the ITC for being too risque). Children watch this show. I'm not in the business of offending anyone.” That's why she got the job in the first place.

The Blind Date concept came from Perfect Match, an Australian game-show. But LWT were worried about the endless innuendo of the pilot presenter Duncan Norvelle and his “chase me, chase me” catchphrase. “My boss said 'Who's the most sexless and inoffensive person on telly?'” says Cilla. “Then he thought of me”. Cilla's always cultivated the good Catholic girl image. She remembers rubbing an Oxo cube onto an orange - a treat in Liverpool, apparently. Halfway through the fruit, she realised it was Friday, and there she was - eating meat. It was the biggest sin she had ever committed. Probably still is. No-one's heard her swear. And she was a virgin on her wedding day. Like the Queen, Gracie Fields and Vera Lynn, Cilla works hard at being a national treasure.

Blind Date viewers these days are more likely to complain about Cilla's outfits than the smut. She's had her mistakes - the trouser suits in purple and ginger (with a slight flur), the maxis, and the disco thing she did back in the 80s - but then didn't we all? She's got Biba and Zandra Rhodes originals in the dark recesses of her wardrobe, but these days favours Escada - so barmitzvah - and DKNY. Well, the DKNY lines that steer clear of latex. According to Stephen Adnett, her costume designer, if Cilla sees something ludicrously expensive - like an £800 blouse - she'll save the show's clothing budget by having him run up a copy. It will fit better anyway. She's not too starry to wear a pair of £15 M&S pumps if an outfit demands it. The end result is very House of Cilla.

The lighting at LWT is known to be harsh on skin of a certain age - all pinks, blues and whites. When she filmed Top Of The Pops at the BBC, Cilla couldn't believe how wonderfully gentle the balance was. An advocate of HRT, Cilla really doesn't look 54 - even under harsh light. But she is trying to grow old gracefully. “I started out as the girl next door” she says. “Then I turned into the auntie next door. I'm fast becoming the grannie next door.” She doesn't like close-ups, and hates having her picture taken. When Wendy, Cilla's regular make-up artist, isn't available for the Sunday Times photo shoot, she panics. The publicist recommends the make-up artist Ralph Fiennes used on The English Patient (“am I really that bad?”). Instead, Cilla prefers to cancel.

Part two of Blind Date, and Marco from Madeira is getting fresh with Cilla. “I've got food in my freezer that's older than you” she says (a variation on the favourite “I've got tights that are older than you”). Marco comes from a little village with a Portuguese name. “Sounds like something the cat brought up” says Cilla. A nice change for her to have an accent to laugh at. She left Scotland Road 33 years ago, but still sounds like a scene-stealing extra from Brookside. “I don't actually think I have got an accent. I've cultivated an accent that people will understand. When I go 'Clur with the fur 'ur', I'm talking posh. Yes. In Liverpool they'd say 'Clair with the fair 'rair'. Posh spice, I am.”

The route from the studio back to Cilla's dressing room used to be marked out with arrows - now it's stills photographs. Otherwise she'd get lost. She kicks off her shoes, uncorks the Moet and changes into her lucky bathrobe. She bought it in a two-for-one special in Woman's Journal 17 years ago. And that means nylon patches on her shoulders. LWT would prefer she opted for the expensive Gloria Swanson number they picked out for her, but Cilla will - resolutely and insistently - be Cilla. It's all part of not forgetting where she came from. “When I started in the business I remember going back to Liverpool and I sat in me Dad's chair. Me Dad had his own chair. 'Eh you - get up' he said. That soon told me.” It only happened once. And that was 35 years ago.

Cilla tidies her hair before popping down to Hospitality. The red doesn't run in the family. At least it didn't until the birth of Jack, her youngest. She's still terrified the colour seeped through her veins. She first dyed her hair when she was 13 - a sevenpenny Camilla rinse from Woolworths. She left it on for three hours, not the three minutes recommended on the box, and the effect on a mousy blonde in search of attention was striking. “I do love redheads” she says to James from South Wales, tonight's contestant number two. James blushes as only a redhead can. He wants to be an actor, and asks Cilla if she's ever done theatre. She nods. 'Really? The National?' asks James. Cilla smiles. Aladdin never made the National. She started at the Palladium, then worked her way down.

Cilla can talk to anyone. Even James. Flying to her villa in Spain last week she met a quiet American. He swapped his Telegraph for her Times, and they struck up conversation. “Over an hour it was” says Bobby, rolling his eyes. “I just looked at the clouds.” James, however, thinks Cilla's attention is down to his boyish charm. “My father said you were some sort of a pop singer.” James isn't to know, but Cilla was on Elvis' jukebox in Gracelands (number 21, apparently). With 19 top 40 singles, she was the biggest-selling British female artist of the Sixties. To anyone the right side of 30, Cilla is just Britain's biggest celebrity. But a new three-CD boxed set, and a Channel 4 documentary about her early pop career should put James' generation right.

The daughter of a docker - that's as good as aristocracy in Liverpool - Priscilla White was raised above a Chinese laundry. English was her best subject at school. Alright, she lost marks for grammar, but she could talk. And, boy, could she tell a story. “We had books at home, but they were all propping up the chimney-breast. My Dad papered over them.” Her final school report read 'Priscilla is suitable for office work'. “But I wanted it to say 'Suitable to be a star'.” At 16, the teachers were right. Priscilla was taken on as a clerk/typist in an architect's office. But Liverpool was swinging (like a pendulum do). It was Endsville. And working as a clerk/typist just didn't cut it. So Cilla decided to get a job as a hat-check girl at the Cavern.

“I got food thrown in for free - tomato soup that tasted like it was laced with disinfectant. Then I watched the Beatles for nothing.” Cilla started staying for the Cavern's all-night sessions - even stepping up to do the odd song. John Lennon told the Beatles' manager, Brian Epstein, about the new girl - and the leathery blackness of her voice. One night Epstein turned up, in a navy blue cashmere coat and a spotted Hermes scarf. He loved her. He suggested the name Cilla Black (Epstein though it sounded earthier), and signed her immediately. Her first single, the Lennon/McCartney composition Love Of The Loved, reached 35 in 1963. Her second, Anyone Who Had A Heart, was her first number one. It's still the biggest selling single by a British female singer.

But there was always more to Cilla than her voice. Some would say just as well. Lennon and McCartney wrote Step Inside Love as the theme for Cilla, her first television series. John and Paul saw their Cilla beckoning, welcoming the British public into her home. It was a three song/one duet kind of show that hung on Cilla's personality. With vox pops and outside broadcasts, it was ground-breaking in a light entertainment kind of way. Cilla still did the cabaret, but the hits were drying up, and presenting became her career. She tried her hand at acting. Work Is A Four Letter Word, directed by Peter Hall, starred 300 chickens and Cilla - the only actor without RSC experience. It was a disaster. But it did teach her more than humility.

“I remember my nose in Work Is A Four-Letter Word” says Cilla. “It was like my profile was in Cinemascope - and I said 'No'. Or Bobby said 'No'”. So she got a nose job for £210 - from the same plastic surgeon as Cathy McGowan. She had a mole removed while she was at it. Then came a professional dry spell. The highlight of the 1970s for Cilla was a voice-over for Cadburys. And Dick Whittington - she still loves an excuse to show off her legs. But on the back of an inspired Wogan appearance to promote her 1983 Greatest Hits compilation, she was offered Surprise, Surprise. And then Blind Date. Cilla was Mrs Saturday Night, on television more than the newsreaders.

Regrets? There are a few. “It does make me sad that concerts now take second place to television. I miss the frocks. That's hard. I do so love a paying audience. I always used to play to the shelf. Not the stalls or the ashtrays (boxes). I figured that the people on the shelf really wanted to see me because the seats weren't that comfortable up there. And from that far away, I can't have looked much bigger than a pin-head. I suppose I am a bit of a snob. I still get upset if the papers describe me as 'a tv presenter'. I'm an entertainer. On my epitaph, I don't want 'Here lies Cilla - the tv presenter'. I want it to say 'Here lies Cilla - the singer'. But I don't need to worry about an epitaph just yet.”

Cilla should have collected her OBE the same day Paul McCartney received his knighthood. But Her Majesty only does OBEs on Tuesdays. And Cilla films Surprise, Surprise on Tuesdays. So the Queen has rescheduled. Cilla had her driver time how long it took to get from the Savoy to the Palace (12 minutes door to door) so she'll be in plenty of time. “The Queen was on a podium - but then I suppose it is her show. The whole thing was done in alphabetical order, and it was a good job I was right up there with the Bs. I wouldn't shut up, and Her Majesty glazed over. Not a nasty look - just 'Oh God, if I don't stop her now she's going to bang on forever.' I can take a hint. It was like visiting your favourite auntie - one who might leave you something in her will. It was great.”

OK! Magazine (“first for celebrity news”) are throwing a £10,000 riverboat party to celebrate. It's a chicken/salmon buffet, with a sponsored OK! cake by Jane Asher. Christopher Biggins, George Martin, Gloria Hunniford - the gang's all here. Apart from Lily Savage. Paul O'Grady (the drag queen's alter ego) is too busy being tied up on a railway line. Don't ask. “I love Cilla” says Paul. “She's still 95% Scotland Road, even though Scotland Road's a flyover now. If people see Maureen Lipman, they think about going over to say hello, but change their minds. With Cilla, they just race over. That's because she's got the common touch. I can't imagine her ever wearing a Jean Muir dress. But I can imagine her soaking her feet, cutting her corns and peeling the spuds. That's Cilla.”

After a four hour cruise (Cilla insists on piloting the Silver Sturgeon under Tower Bridge), Bobby and Cilla drive home. Not to the vulgar pile you might expect - with rouched loo-paper and shagpile so thick you could trip over it. It's all mint green sofas, and gentle pastels. With Lowry on the walls. And a 10-seater Carolean table in the dining-room. On the sideboard stand Cilla's best presenter award from the Royal Television Society (“that's so snobby they don't even televise it”), her Bafta for Blind Date, and two lifetime achievement awards. Now she has to find room for the OBE (that's one word, rhyming with Moby). “It's a dusty-pink ribbon with white stitching. I think it'll go well with the salmon-pink colour scheme we've got in the dining-room.” It's one bit of dusting Mrs Willis won't mind doing.