Family Tree, BBC Two

Christopher Guest and his group of players have been responsible for some of the funniest, driest comedy films of the past 30 years, including Waiting For Guffman, Best in Show, A Mighty Wind and, of course, his masterpiece This Is Spinal Tap, in which he played the tight-trousered guitarist Nigel Tufnel. Now he's directed and co-created (with Jim Piddock) Family Tree, a US-British sitcom first shown on HBO in America.

Like most of Guest's work, the mockumentary sitcom is improvised by the cast and it centres on Irish actor Chris O'Dowd's Tom Chadwick, a 30-year-old unemployed claims assessor who was dumped by his girlfriend six months ago. His sister, Bea, is played by British ventriloquist Nina Conti, their disparity in accents explained by the fact that Tom moved with his Irish mum to Ireland after their parents split up, while Bea stayed with dad Keith (a strong performance by Michael McKean, one of the Guest ensemble) in England.

Conti appears with her puppet Monkey; after being traumatised by seeing a puffin masturbating while on holiday in Wales as a child, Bea was advised to use a hand puppet to express her inner thoughts and emotions, although not in a Mel Gibson-The Beaver kind of way, as both Bea and Monkey talk. Monkey provided most of the laughs as he disparaged Luba (Lisa Palfrey), their dad's incomprehensible Moldovian wife, and her atrocious cooking, which always involves sausages, even in a Sunday roast. "It's inflatable," Bea says of a bouncy castle. "Like your wife," says Monkey.

Tom has just inherited a chest full of bric-à-brac from their recently deceased great-aunt Victoria, and so starts his quest to uncover his family tree, which will later include several American relatives, aided and abetted by his best friend Pete Stupples (Tom Bennett), local junk shop owner Glenn Pfister (Piddock) and history expert Neville St Aubrey (Christopher Fairbank).

There are some neat visual gags. Tom lives above a pet shop called Macaw Blimey, and the television that's always on in his flat or his father's house shows spot-on spoofs of British TV - a cringeworthy 1970s Asian sitcom with Meera Syal (“There Goes the Neighbourhood”) and an overwrought costume drama (“The Plantagenets”).

But overall the pace is glacial and there are lots of oddball characters, seemingly with the sole purpose of allowing the actors to give a performance rather than to achieve some characterisation, while neither O'Dowd and Conti grip one's emotions. I wish I could say this opener was laugh-out-loud funny, but I can't. Here's hoping, though, that over the next seven episodes the Guest magic (including an appearance by him as a long-lost Chadwick relative) will start to work.