The IT Crowd returns for a one-off special which will most likely be its last ever (since Richard Ayoade is now a director and Chris O'Dowd is Hollywood's most unlikely heartthrob). Despite repeated attempts, I've never managed to find it even the slightest bit amusing - its strained "aren't geeks weird!" jokes and broad performances, accompanied by gales of studio audience laughter, just aren't for me. But for those who have enjoyed the previous four series, the special offers more of the same, wisely keeping to the same basic premise (and basement) rather than sending them all on holiday or something.

It's been a rum little monkey, Family Tree (BBC Two), full of bad jokes and oddball characters.

But, as Chris O'Dowd's Tom Chadwick dithered over his future in a 'there must be a series two'-type cliffhanger, there was a surprising melancholy to waving farewell to this curious bunch at the airport.

The satire on the whole Who Do You Think You Are? TV franchise was a tad overcooked - surprisingly so, given the pedigree of writer/director Christopher Guest - but there was just enough wit scattered among the branches of Tom's extended clan to make up for the feeling that no family could contain this many nutters.

'Do you find that being around books makes you more clever?' Tom asked potential squeeze Ally. 'No, it makes me feel like all the ideas have been written already,' she replied. They were made for each other.

And Fred Willard's Al Chadwick had a camp hoot with a stream of bad gay jokes. 'How do you fit four gay men on one bar stool?... turn it upside down!' So wrong.

Tom (Chris O'Dowd) is still in California with his new-found relatives in Christopher Guest's transatlantic mockumentary. The Chadwick clan's investigation into why their ancestor Charles fought for both sides during the American civil war is punctuated by Tom's startled amusement at his extended family's pasted-on quirks, because why bother with actual personalities when you can give your characters a gently eccentric passion for historical re-enactments (or conspiracy theories, or owls) instead?

This is a hit-and-miss sort of sitcom at the best of times; this week it becomes predictable in the wrong way. When we hear that our hero Tom's great-aunt left her home to a female friend, we wonder if they were actually more than friends, but Tom's dozy friend Pete takes ages to twig. Also, one minor character is a gym owner whose West Indian patois is hard to understand: isn't that the kind of joke the series mocks in its 1970s sitcom pastiches? Anyway, Chris O'Dowd is still good as Tom and has a nice monologue about being single.

With star Chris O'Dowd and This is Spinal Tap creator Christopher Guest attached, there has understandably been quite a bit of hype and expectation surrounding this new comedy series. Thankfully, three episodes in and that hype is looking increasingly justified.

It's difficult not to draw comparisons to the creator's previous comedy ventures: while some of the gags are groaningly predictable (as when cocky Pete is pitted against a small child in the boxing ring - what could possibly happen?), the very best jokes in the series are the deadpan, subtle 'did I just hear that' variety that made ...Spinal Tap so funny. Tom's ventriloquist sister Bea (Nina Conti) is particularly hilarious, her id-outlet monkey hand puppet sharp, rude and wonderfully weird. Their appearance this week as wedding entertainment is predictably disastrous.

Meanwhile, this episode takes Tom's bittersweet - and rather hopeless - search for his family history into the world of sport after he makes another discovery. Hopefully those discoveries will keep on coming as this is getting better by the week.

There's another lovingly detailed pastiche of a 1970s sitcom right at the start of tonight's episode. Tom (Chris O'Dowd) visits his father to show him the photo of their ancestor, but first has to distract him from watching DVDs of his beloved Move Along Please!, a police station comedy that's a sort of Carry On version of The Thin Blue Line ("Well you said you wanted us to make a big bust this week, Sergeant!").

It's one of many lovely touches in a comedy that looks like being a grower. The slightly dopey Tom has embarked on a genealogical quest that takes him to a theatre in Hove, where the over-reverent historian ("I look on the theatre as like a temple...") is beautifully played by Hugh Sachs from Benidorm.

It turns out Tom's great-grandfather was an actor - and he played opposite Laurence Olivier, no less.

In the new sitcom Family Tree, Eeyore-faced Chris O'Dowd plays Tom Chadwick, a recently cuckolded, jobless single who's inherited an old photo of someone he believes to be his great-grandfather. Tom embarks on a search to know more about his ancestors, discovering ever more exotic and esoteric branches of his genealogy. He's accompanied by his hapless pal Pete and sister Bea (the ventriloquist Nina Conti), who, due to a traumatic childhood incident in a zoo, now voices her more unorthodox opinions via a hand puppet called Monkey.

This is a very funny comedy and, watching preview episodes on my computer with headphones on, I chortled out loud about six times (a risky thing to do when colleagues all around are ploughing through an Ed Miliband speech). The series is a send-up of kin-hunting series such as Who Do You Think You Are? and Long Lost Family, and because tracking down one's forebears could lead to just about anything, both possibility and improbability are built in. As a documentary spoof, it's more farcical and less 'realistic' than, say, The Office. Nobody really questions the presence of Monkey.

Conscious of its own meta-ness, the show makes fun of different genres, often via its characters watching telly or DVDs, so we get treated to snippets of a mock BBC2 historical soap, or a cringe-inducing flashback to a 70s sitcom. One thing about these TV-within-TV moments, though. It's become increasingly the done thing, in our overly politically correct times, for hip, knowing comedies to get safe laughs by showing terribly un-PC things via the filter of a terribly un-PC character (in this case, Tom's dad). We're supposed to be laughing at, not with, the character. In reality, of course, we're doing both. I find the PC brigade tedious in the extreme, but it seems to me that, if you believe one shouldn't depict Indians with waggly heads, then don't depict them, full-stop. You can't poke fun by proxy. That's having your gluten-free cake and eating it too.

Co-written and directed by Christopher Guest, pioneer of the mock-documentary format, Family Tree stars Chris O'Dowd as a man intent on tracing his family history.

The performances are naturalistic, the pace leisurely, the humour gentle, the focus meandering and the format flexible enough to include mock-doc TV interviews - despite it not being set up as a documentary - and for Nina Conti, who plays O'Dowd's sister, to employ her ventriloquist puppet monkey as a co-star.

The show is amiable, entertaining, whimsical and intriguing, but - a terrific blind date scene notwithstanding - it doesn't seem that bothered about being funny.

When it comes to top pedigree, Family Tree (BBC Two) is hanging off some high branches. Boasting Chris O'Dowd, a big (well, at least medium) Hollywood cheese these days, as the central character and Christopher (Best In Show) Guest on writing and creating duties, you might expect this to be a big deal movie release. Instead, it's an oddly understated sitcom.

O'Dowd is a fresh and engaging comic talent. He breathes believable life into Tom Chadwick, one of life's drifters, whose aimless existence takes a surprising turn when he's left a chest containing random bits and bobs by a long-forgotten great aunt. Among the bobs is an old photo of a military gent, which sets our Tom off on a Who Do You Think You Are?-style jaunt.

The trouble is, everyone else in Tom's life is mildly bonkers, from his ventriloquist sis to the crackpot in the corner shop. Tom Bennett is a notable exception, good value as best mate Pete, but he has to fight his way through the gaggle of eccentrics choking Family Tree at the roots. And that confessional speaking-to-camera trick? All too wearily familiar.

Family Tree is an odd kind of affair. It's been created by comedy aristocracy (it's co-written and directed by Christopher Guest, who created This Is Spinal Tap), but it came across in this first episode as amateurishly awkward, funny in a desultory kind of way at one moment, startlingly clunky at others.

Chris O'Dowd stars as Tom, an out-of-work risk assessor triggered to research his ancestral past by a bequest from a great-aunt, and Nina Conti plays his sister Bea, complete with the monkey vent doll, explained here as the result of childhood therapy for elective mutism ("she hadn't skoken in weeks," the monkey helpfully explains). The dialogue has the loose, bantering style of improvisation - which gives it a warmth and realism to counterbalance the slightly effortful zaniness - and the style (in a half-hearted way) is mock-documentary.

The format doesn't make much sense. Why would anyone be making a film about Tom and his family, particularly since the mission to explore the past hasn't even occurred to him at the beginning of the episode? Besides, there doesn't seem to be any real tension between the actuality sequences and the more formal talking-head interviews that occasionally pop up (in the style of Modern Family).

But it is Christopher Guest. Some of the character comedy, promisingly, is funny, in particular a sequence in which Tom went on a blind date with a very stupid girl ("There's been loads of sightings of dinosaurs in Africa," she assured him when he expressed polite doubt about their continued existence). Watch, in the hope of developments.