Last Night's Television: Big Top, BBC 1The Man Behind the Masquerade, BBC4

Big Top, a new sitcom set in a travelling circus, is one of those programmes that get you wondering about the commissioning process. You'll need something to entertain you while it's on and speculating about the way it came into being will do as well as anything, unless you've got a dog that's overdue for a combing or some socks to pair up. One assumes that the performers' names came first on the pitch document. One certainly hopes that they came first on the pitch document, since the idea that it was sold on the essential concept and a sample of the writing seems implausible, to say the least. We've thought of a vehicle for Amanda Holden, somebody said, and what's more it's a role that will make it feasible for her to wear hotpants and black stockings nearly all the time. And if you bite there's a good chance that we can bolt on John Thompson, Tony Robinson and Ruth Madoc. How's that for belt-and-braces coverage? Cold Feet, Blackadder and a dab of Hi-De-Hi! behind the ears.

"So what's the sit?" asks the commissioning editor. Down-at-heel circus, replies the pitcher, run by Lizzie, a mildly over-controlling ringmistress who's the only grown-up on payroll. There's a terrible husband-and-wife clown act, a depressive East European acrobat with a crush on Holden's character, a cynical soundman called Erasmus (Tony Robinson) and the self-seeking Welsh dame who does a performing-dog routine. Oh, and it's written by Daniel Peak, who wrote Not Going Out, so there's a bit of pedigree there. Lot of running gags, lot of slapstick, comedy of types. Think Dad's Army with red noses and spandex tights. And then, one guesses – since it's not very often these days that sitcoms get green-lit without jumping through this particular hoop – there would have been a rehearsed reading of the script, so that a collection of executives could mull over its prospects. And it's at this point that speculation hits an obstacle. How could they sit in the presence of gags this lame and character depiction this arbitrary and not say no?

It does go out at 7.30pm, so it's possible that the younger audience will be advanced as an alibi. It seems heartless to use children as a human shield in this way though, and surely they deserve better than gags about ferrets down trousers and punch lines that audibly creak as they're winched into place. "I was so worried that you'd fail us on the raw sewage round the hot-dog stand," blurted out Lizzie when the health and safety inspector gave her the all clear, a line that not even Helen Mirren could have made psychologically plausible. And without an underlying psychological plausibility (the urgent cartoon drives that you'll always find in Hi-De-Hi! and Dad's Army if you dig deep enough) it just isn't comic. That line isn't an inadvertent revelation – it's hopelessly, mechanically advertant, only there to be funny. In the end, an exchange between Plonky the clown and Erasmus offered the best verdict: "If we're so terrible why do we get a big cheer when we finish?" "I think you've answered your own question there."

The Man Behind the Masquerade, an affectionate portrait of the artist and illustrator Kit Williams, had one really shrewd stroke in it. Somebody had noticed that there was a connection to be drawn between Williams – who conceived and created the Seventies treasure-hunt sensation Masquerade – and the Turner Prize- winning artist Keith Tyson, whose paintings often employ a similar idiom of brightly coloured, photo-realistic detail. Tyson frequently gets studio assistants to help paint his giant canvases while Williams does everything himself, including manufacturing the props and the elaborate marquetry frames. He lives in what appears to be a life-sized Lilliput Lane cottage in Gloucestershire, with a wife who makes "narrative jewellery" (search me) and produces paintings of a ruralist eroticism, in which pert young girls share the frame with goatily intrigued older men and botanically precise bits of flora. He'd been so shocked by the global sensation caused by Masquerade – which offered heavily coded clues to the location of a buried treasure – that he'd retreated from the world for 30 years. But having pushed the cup of publicity from his lips he'd come back for a little sip and seemed – on the evidence of his sparkly demeanour at his private view – to have enjoyed it enormously. Somebody called him a genius at one point, which seemed to me pushing it a bit. What Tyson has and Williams doesn't – paradoxically given what made his name – is a sense of serious enigma in the art (Williams's youthful paintings, less dependent on reference photographs, look far more interesting than the recent work). He is a kind of genie though – a presiding spirit of place who can conjure dazzling things into existence – and on the evidence of this film he's a lovable genie too.