Coulrophobics beware: this episode features an exceptionally weird clown sequence in which Mr Jelly (Reece Shearsmith) is tried in "clown court" for bringing his profession into disrepute. His punishment appears, initially, to throw some light on what's happening in this increasingly bizarre (Christopher Biggins in a gold lamé codpiece is the least of it) black comedy but, true to form, the clue proves unreliable.

There was only one hitch with The League of Gentlemen - it just wasn't weird enough. Which is why League members Reece Shearsmith and Steve Pemberton have invented another unusual town, which is home to a deranged midwife who looks just like Dawn French, a blind recluse who has an unusual hobby, a telekinetic dwarf, a hook-handed clown, a serial-killer obsessed man-child and, er, Christopher Biggins.

It's a bitterly cold November morning in a Mill Hill seminary, so cold that it's hard to distinguish the mist and dry ice from our breath. St Joseph's College was founded in 1866, and Steve Pemberton, looking of a similar vintage under heavy make-up, is rocking a ZZ Top in Middle Earth look, with wispy beard and satin gown. He's also lost a wart, but in between searches he sits down to chat about new project Psychoville. Fellow The League of Gentlemen alumnus and co-creator Reece Shearsmith has also stopped by, thankfully in civvies rather the outre garb of some of his characters.

The show, both concede, is unavoidably reminiscent of Royston Vasey in its grotesque characters, reference-heavy humour (episode four is a Hitchcock tribute, shot in just two takes in the manner of 'Rope') and even the name; Psychoville is the title used for The League of Gentlemen in Korea.

But there's no sense of recycling ideas. The characters may be oddballs, but they're driving a crafted narrative and are played by a broader cast. There's disturbed midwife Joy (Dawn French, right), misanthropic kids' entertainer Mr Jelly (Shearsmith), telekinetic dwarf Robert (Jason Tompkins), serial killer-obsessed David (Pemberton) and his mum, Maureen (Shearsmith), and Mr Lomax, a blind recluse with, yes, a straggly beard (Pemberton). Eileen Atkins and Christopher Biggins are also among the deliciously eclectic on-screen talent who've at least had the courage of their convictions. 'We've had famous people saying: "I'll do anything, I'll make sandwiches."' laughs Shearsmith. 'Then you offer them a part and they say: "Not for me."'

The co-creators' curious real-life experiences once again bleed into the characters. 'Joy the midwife came from sitting with glum dads in antenatal classes,' says Shearsmith, with appropriate gloom. And Mr Lomax? 'I used to work for a blind man while I was at college,' remembers Pemberton. 'I thought I'd be reading Dickens and we'd do crosswords together. I ended up writing his cheques and reading out financial reports. If there was a graph, I'd leave it out - how do you describe a graph to a blind person?'

Most grimly, David was 'the friend of a friend. He was jaundiced and his mother only had one leg. Someone went to their house and saw these bottles of wee from where she couldn't get to the toilet; she was scratching her son's back with her foot. As an opening image, that's terrific!'

Horizons are broader than they were in The League, with five locations forming the backdrop to the stories of the main characters. Each setting, from Wood Green to Ilkley, has its own tonal palette (London's is orange, to fit David and his mum's curtains - and skin).

And gradually, the plot brings these disparate, scattered characters together to reveal the secrets hinted at when each receives an anonymous letter claiming: 'I know what you did.' The mystery is all the more enveloping as neither writer knew how it would end. 'We were writing ourselves into corners,' says Shearsmith. 'But if we didn't know how we were going to untangle ourselves, then hopefully the audience can't second-guess us.'

Later on, we join director Matt Lipsey in the catering truck, where he's chuckling wearily about episode four. 'The scripts are so detailed, it makes my life hell, but it's the complexity that makes the show. That 'Rope' episode was painstaking, but it was exciting for the actors: as they got closer and closer to completing it they got more and more nervous about screwing it up. Which one of them did on take 15 ...'

Psychoville is evidence that there are commissioners at the BBC willing to take a risk in a conservative climate. 'Anything that smacks of a diktat from on high is dangerous,' Lipsey argues. 'Surely it's about diversity.' Pemberton and Shearsmith concur. 'When we wrote this in 2006,' says Pemberton, 'we heard that dark was out and big and funny was in. But we ended up being drawn to the things we were drawn to anyway.' And, rather than conduct the first read-through in a White City conference room, they did it in front of 250 people in a Notting Hill theatre. 'We wanted to hear the sound of laughter so they didn't just think it was something dark and weird that no one would find funny,' he beams.

But, creepily hilarious as Psychoville undoubtedly is, one question still hangs in the air: what about a League reunion? 'The will is there,' begins Shearsmith. 'There's no reason why not,' adds Pemberton, before wandering off into the chill in search of another wart. Trust us, saying 'Hello David' for the next few weeks is no bad thing.

Created by two of the stars of cult hit The League of Gentlemen & exhibiting the same kind of dark, left-field humour, this new comedy-thriller is skilfully made but certainly not for the faint-hearted. Its begins with a mystery, as five weird characters - among them a deranged midwife, an embittered clown, and a blind millionaire - each receive a card containing the anonymous message 'I know what you did'. The ensuing narrative involves blood, bad language & Christopher Biggins and is, in its own macabre way, highly entertaining.