TV review: Princess Diana's Dresses: the Auction, Channel 4

"Every sequin, stitch and seam provides a window on to her short but extraordinary life," said the voiceover at the beginning of Princess Diana's Dresses: the Auction, an instructively besotted film about the sale of 10 of Diana's frocks. "These dresses," it had already promised, "hold the key to understanding Diana as never before." An hour later, with your understanding of Diana unchanged by a single stitch, any doubt about that claim had hardened into certainty.

Which didn't mean the film was entirely pointless, though. Because, as the Duchess of Cambridge trembles on the brink of delivery, it provided a timely reminder of the masturbatory hysteria of the British press when in pursuit of a royal story. If Kate is still unsure about how over-excited, vicious and inconsistent it can get, this should put her right.

The sale here wasn't the big 1997 auction Diana held herself, to offload old schmutter and raise money for charity, but an ancillary sale conducted by Kerry Taylor, who testified to the unsettling fetishism of the collectors when it comes to acquiring Diana relics. Some of them actually get the dresses DNA tested (is her DNA on public record, then?) and none of them would dream of washing them. A wine stain, a heel mark, or the ghostly imprint of childish fingers in green velvet all add extra value, and could, I suppose, be regarded as delivering some kind of biographical detail. Apart from that, these dresses, all from the later stages of her time in office as the paparazzi's number one target, couldn't supply a lot in the way of revelation.

Instead, they supplied an excuse to tell the old familiar story, of the frump who kissed a prince and turned into a fairy-tale enchantress. The prince, meanwhile, turned into a grouch, petulantly sulky at the fascination his wife exerted over the members of the press. And the most interesting dress here was one that wasn't actually in the sale – the Christina Stambolian black number that Diana pulled out of the wardrobe on the night Prince Charles talked about the marriage on television for the first time. If he'd thought he might command the front pages for one day at least, he severely underestimated her. She was both bombshell and delivery system all in one, and blew him into the side columns.

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.