When is the wet shirt moment? "There isn't going to be one," says Andrew Davies. "There are some very tender glances in Little Dorrit, but there is no 'Phwoar!'"

Can this be true? A straight take on Charles Dickens from the man who put the cor blimey into costume drama by having Mr Darcy rise from a lake in a soaking, see-through shirt? Not quite. His lavish adaptation of the debt saga, which starts tonight, has been heavily hyped as a Victorian version of the credit crunch, but it also features a beast of a man so magnificent that corsets will tremble. Even if Rigaud does love 'em and leave 'em a bit too literally. "His first words are 'sacre bleu!'" says Davies with a laugh, admitting the wild-eyed, piratical Frenchman is way over the top. "I thought, 'Why not?'"

Then there are the lesbians. What, you don't remember them in Dickens? You weren't paying enough attention. "You come across Miss Wade," insists Davies, "and say, 'Wow! She's a lesbian. Got to be!'" He grins. Even in his 70s, he loves mischief. Dickens doesn't actually say that, though, does he? "No," he admits, "Dickens doesn't say that, but he is probably implying it."

Probably? This is the sort of thing that gets his critics really annoyed. Andrew Davies is obsessed with sex, they say. He slips it in where it has no place. "It's curious," Davies says, leaning back and putting his feet on the table. "The whole sexuality thing in Dickens is tremendously tamped down. It appears as love and tenderness and pity, which is very interesting. He has pure and lovely heroines, whereas we know that in his own life, what Dickens wanted to do with girls like Amy Dorrit was to fuck them."

Did he? "He did." They didn't teach that at school. "No." Before he was a writer, Davies was an English lecturer, and he does address you as if giving a friendly tutorial – but his greatest achievement in getting people interested in the classics came with the rewrite of Pride and Prejudice for television in 1995, for which he created a gorgeous, thrusting Darcy.

He hasn't had a flop since, so to speak – apart from Brideshead Revisited. Questions on the latest version of the Evelyn Waugh novel are not very welcome. It has been mown down by the critics like Charles Ryder's schoolchums in the war. "I can get out of that by saying there's not much of my script left in the actual movie," says Davies. Why not? "I can't talk about it." Oh, but he will. The look in his eye says so. Actually, he has a way of staring when he's talking to you that is unnerving, like an undertaker sizing up a body. Or a writer looking for material. His crumpled blue linen suit is presumably for the photographer and makes him seem less at home than he should be in his own house in Warwickshire, with books piled high around him. Little Dorrit, he says, was "such a cumbersome book I cut it in half". Literally? "Yes." He makes the sign of scissors. "It was a Penguin classic. I read the first half, then read the second. I should have cut it into quarters, really."

See? The critics will hiss that he just doesn't care about books. The rest of us have no need to feel guilty for not having quite managed to get to the end of the 800 or so pages yet. He has turned them into a one-hour special tonight, then half-hour soap-style episodes to be shown twice a week. As usual for him it is studded with stars – Tom Courtenay, Matthew Macfadyen and so on – it looks gorgeous and the resonances with modern life have been turned up to 11.

Amy Dorrit is born and raised in the Marshalsea debtors' prison, and everyone else is drowning in debt too. "There is a bank that goes bust," says Davies. "Honest businessmen are struggling to achieve anything, but meanwhile money is being mysteriously conjured up out of nothing in the City. Everyone pours their money into that, then it all goes disastrously wrong." Sounds horribly familiar. "Yes. It is very good timing. It is fortuitous..." But not accidental, somehow. He has an uncanny knack of making historical dramas look as if they were written yesterday.

Of course, bits of them were. By him. Such as the subplot that sees the downtrodden Tattycoram lured away from her wealthy adopted family by the enigmatic Miss Wade. "They have this very intense relationship," Davies insists. "There is some kind of emotional dom-sub thing going there between the two of them." He has turned the "implication" that Miss Wade is a lesbian into something far more direct, although "the most we see is a kiss, and a chaste one at that".

They don't go to bed. Nobody does in Little Dorrit. You would think he had calmed down, but for the immense sexual tension between Amy (Claire Foy) and the much older Arthur Clennam (Macfadyen), her would-be saviour. "Delaying the moment where they touch or kiss just helps tremendously," Davies says, as if this were a radical new way of writing, not something Austen did first. "You're thinking, 'Come on Arthur. Can't you see she's up for it?' In her dewy, innocent way."

Right, so, an older man falls for a young woman? That may have been autobiographical for Dickens, but what about Davies? He has been married to Diana since 1960, but once said fidelity was "something to strive after". He has admitted often over the years that he dreams of affairs with women 30 years younger – and dropped plenty of hints that this may have happened. "Have I said that?" He knows he has. "Well ... look ... I'm 72 now. I think I might have got over that."

People do think of him as a horny goat. (He did co-write the laddish sitcom Game On.) "People can make whatever assumptions they like. No, these days I think about food and wine much more often." Except when he's writing. "The biggest thrill I get is probably far more perverse and kinky than anything else. I actually like imagining myself as an 18-year-old girl."

Pardon? "I don't mean I'm going to nip upstairs and put my frock on." Oh. Feel free. "No, but when I'm doing this work, I just love putting myself in her head and thinking up new bits of dialogue for her."

Known for writing great women, he says he has "a huge amount of help" from his wife and colleagues at the BBC. "I had a mother who was very emotionally demanding, wanting to be the centre of attention. As they say in EastEnders, she thought it was all about 'er. I spent a lot of time trying to work out what was going on. I was also fascinated by girls from an early age." He had dolls, and during war games "preferred to go to the hospital to have my wounds tended by girls".

Actors love his stage directions, which are often "quite anachronistic". He laughs at the thought of Claire Foy, who plays Amy, trying to get inside the mind of a Victorian in rehearsals, then reading the direction that tells her what look should be on her face: "'Oh fuck,' thinks Amy, 'I wish that hadn't happened.'"

Think about that when you watch. She's good, but the show may be stolen – particularly in episode two – by Andy Serkis as Rigaud. "He's brilliant," Davies agrees, "and creative. He kept suggesting extra bits of business for himself, like the tiny subplot in which he seduces a French landlady. That didn't appear in the original script." Not in the book? "That's right." So Davies was being played at his own game, improving the classics by making sexy stuff up? "That's right." How can he mind? He does it all the time to the authors. "The author doesn't necessarily completely own the book. It's an interaction between them and readers at a different time."

You would expect the king of costume drama to live in a mansion at the end of a long drive. There should be a carriage at the door, with two lovely black horses ... not a used car salesroom opposite, and an alarm blaring. This is perhaps the most startling thing about Davies, one of the most acclaimed writers in modern television who is reputed to earn at least £200,000 a series: he and Diana live not in Hampstead or the country but in two houses knocked together, on a busy street near the centre of Kenilworth in Warwickshire. They moved to the area when he became a lecturer at Coventry College of Education, which was merged with the University of Warwick. He did not leave the job to write full-time until 1987, when A Very Peculiar Practice had become his first hit. There were others, but adaptations such as Middlemarch, Moll Flanders, Vanity Fair and Bleak House became his speciality. They have never moved away.

"I suppose I have the tastes of someone who teaches at a university in the provinces," he says, "rather than – as Alan Bleasdale used to put it – some arty farty literary wanker." What tastes? "Modestly sort of sitting at home, drinking a bottle of wine with my wife and watching telly, or seeing some friends. Rather than going to someone's sodding book launch, surrounded by a whole lot of screaming people."

Kenilworth was nice place to bring up his two children. "Our kids used to complain how boring it was. We'd say, 'Good. Do your homework.'" They did. His daughter is a script editor in Yorkshire; his son teaches acoustics at Salford University. "He's a scientist. A clever geezer. I'm very impressed."

Davies prefers screenplays to novels because you don't have to produce pages of description, just a telling image. So what would that be, if he was going to write about Kenilworth? He sighs. "Oh God. I don't think I would set anything in Kenilworth. It's a place to live in. It's not a place to write about."

It's a place to retreat to, which may have been useful after the Brideshead experience. Davies is credited as having written the screenplay with Jeremy Brock. "When you see two writers named on a movie, one of them did some drafts and got the boot."

Why did that happen to him? "Um. Well, the polite word was that Andrew wasn't available because he had moved on to other things. The truth was that Andrew wasn't asked whether he was available to do any more drafts, for reasons that I never knew, really."

How would his version have been different? "I think people might have liked it even less." He wanted far less "swanning about in nice white flannels in Oxford". The heart of the book is Julia's rejection of Charles, he says, not his relationship with Sebastian. "The problem is that I have signed a contract to say that I can't say anything bad about the movie." Other people have said some terrible things. "Mostly," he says. "Yes." A critic once said he crawled inside a classic like a hermit crab into a shell, and walked off in it. "That was not meant to be a compliment, but I love it... although it looks as if I didn't get away with it this time. Oh well." Why should he care? The late-flowering career of Andrew Davies can take a blow like that. As one of his heroines might almost say, looking at the groaning shelf of DVDs: "My dear sir... look at the hits on that. Phwoar."

Highlights

A selection from Andrew Davies's film and television credits in a career spanning more than four decades.

1965 'Who's Going to Take Me On?'

1976 'The Signalman'

1980 'To Serve Them All My Days'

1986 'A Very Peculiar Practice'

1992 'Moll Flanders'

1994 'Middlemarch'

1995 'Pride and Prejudice', starring Colin Firth as Mr Darcy

1998 'Vanity Fair'

1999 'Wives and Daughters'

2001 'The Way We Live Now'

2002 'Dr Zhivago' and 'Tipping the Velvet'

2004 'Bridget Jones: The Edge of Reason' (with Helen Fielding)

2005 'Bleak House'

2006 'The Line of Beauty'

2008 'Brideshead Revisited'