"Some things will upset the purists - that's inevitable," Marchant shrugs. "But the weird thing is, if I don't upset the purists, maybe I haven't done a good adaptation. The mark of a good adaptation is how many letters you can attract from the Charles Dickens Society."
Letters may well come flooding in about Marchant's portrayal of Miss Havisham. As played by Charlotte Rampling, she is a borderline sociopath. Twitching, half-smiling, distraite, she whispers to the young Pip (Gabriel Thomson) when she first meets him: "I sometimes have sick fancies." Having been humiliated by her and her protegee Estella, Pip runs tearfully from Satis House, all the while conjuring up gruesome visions of Miss Havisham hanging from a rafter. This Miss Havisham is what a 1990s analyst would call "dysfunctional".
"She is ripe for psychotherapy," reckons David Snodin, the producer of Great Expectations. "In modern terms, she's a quintessential clinical depressive. She's an agrophobic who won't wash. These days she'd be on Prozac - and it wouldn't do her any good. Dickens purists might want a larger-than-life Miss Havisham, but Charlotte's is more frightening because you haven't the faintest idea what she'll do next. She brings a very contemporary sense of madness to the role."
Nearing 40, Marchant is a benign and unfeasibly tall figure who has to stoop when he passes through a doorway. Brought up on a council estate in Bermondsey, he used to spar at the famous Thomas a Becket gym in south- east London. "At first Tony didn't know whether to be a boxer or a poet," laughs Snodin. "He thought the theatre was full of poofs in cravats."
Marchant went for the cravat option and now resides with his wife and children in a comfortable house in south-west London, whose walls are adorned with awards and posters from his successful series. The one advertising Different for Girls, his feature film about a man who has a sex-change operation, boasts the catchline, "expect the unexpected" - which might stand as a summary of Marchant's emotionally raw brand of drama.
He argues that you have to take account of modern sensibilities when interpreting Great Expectations; you can't pretend the 1990s never happened. According to Marchant: "It's important to pay attention to the psychological motivations of the characters. For instance, I've looked at the idea of nurture and exactly what Miss Havisham wreaks on Estella. I couldn't help noticing that this is a book about abusive relationships. Abuse continues into other relationships. The fact that Estella ends up in a relationship where she's abused follows the received wisdom of modern psychology that there's a cycle of abuse."
It is these contemporary echoes that distinguish Marchant's adaptation. (They are also what attracted John Sullivan to dramatise David Copperfield for the BBC and Alan Bleasdale to pen his ITV version of Oliver Twist). One of the abiding themes of Great Expectations is class - a subject which, like the poor, is always with us.
"You could say Pip's preoccupation is the same as Tony Blair's - how we all want to become middle class," Marchant surmises. "Pip thinks: `How can I run away from my working-class existence and be appreciated by my betters?' The contemporary corollary would be: you move to Islington and acquire a taste for balsamic vinegar.
"The idea that improvement is measured in material terms is part of the political currency now. Pip and Estella's idea of progress is financial independence, and I see parallels with that today. Also, Pip's whole ambivalence about his humble background is - depressingly - still very potent."
More than anything else, however, Great Expectations chimes with Marchant's overriding interest in our contradictory natures. This was seen previously in the characters of Shaun (played by David Morrissey), the Inland Revenue inspector from Holding On who turns to embezzlement, or of Roy (Alun Armstrong), the grief-stricken carer in Goodbye Cruel World who starts to steal from the charity he administers.
"There has always been a proximity between criminality and respectability," Marchant says. "Without being too zeitgeisty about it, look at the story of Jonathan Aitken. It's that whole thing about turning up a stone to see what's underneath. For instance, what Pip imagined to be the great and the good turn out to be anything but. Think of the way Mr Jaggers's reputation is diminished in Pip's eyes when he is revealed as a morally contemptible figure. We all look at people like that and wonder what's in their cupboard."
These ambiguities exist within us all. "We're constantly confounded by the paradox between what we think we ought to feel and what we actually feel," Marchant continues. "That's what drama should be about. Drama is about aberration and conflict, and conflict comes when we don't quite add up to what we profess to be. Pip is full of those contradictions."
For all the modern resonance of Great Expectations, isn't there still a danger that viewers will groan: "Oh no, not another period drama"? Marchant thinks they should only complain about bad period dramas. "As long as they're done well, they're worth it. It's always worth revisiting Great Expectations, because every generation can bring something fresh to it. No one says to the Royal Shakespeare Company: `Why are you doing Henry V again?'"
Marchant explores comparable dilemmas in Bad Blood, a new three-parter for ITV about the moral disintegration of an infertile surgeon (Alex Jennings) who resorts to desperate measures in his quest to adopt a Romanian baby. It again coheres with the writer's ideas about social facades.
"It's about how private inadequacies reflect themselves in public acts," Marchant says. "When I was growing up, I was knocked out by the writing of Arthur Miller. It was revelatory to me that something which is morally dense can be exciting at the same time.
"You can't ask people to buy into the more rarefied things you're trying to achieve unless you've grounded the story in a reality we can all recognise. Otherwise, it would just be a thesis. So when the Alex Jennings character starts doing things that are beyond the pale, we have already located ourselves with him emotionally. It's more disturbing to say: `I understand exactly why this guy has gone on this journey.' It's the Macbeth Syndrome."
Do not, however, come to Marchant if you're looking for happy endings; he cheerfully recalls "blubbing away" in a public library while writing Goodbye Cruel World. He is currently working on a "heavy" three-parter about the effect on a family of a boy with attention deficiency hyperactivity disorder. "Stoic endings are the best I can manage," he says. "Drama is not about people being happy, it's about people being miserable. It's about things not being normal.
"It's important not to skimp," he concludes. "I want to go as far as I can without people switching off. In Goodbye Cruel World, there was a scene where a son carries his seriously ill mother to the loo. The question was at what moment the loo door would be shut on the camera. But we didn't want to be tastefully discreet. We wanted to confront the reality of a son coping with his mother's disability. In the end, we kept the door open the whole time." Brace yourselves for a similarly uncompromising experience with Great Expectations.
`Great Expectations' is on BBC2 on Mon and Tues. `Bad Blood' is on ITV on 18 April