When will someone make a documentary about documentary-makers - a fly on the wall of the fly on the wall? Watching television last week, I saw people being brainwashed into calling vacuum cleaners "complete homecare systems", a man wiping the nose of a greyhound statue, and women having their faces peeled off as they lay on their backs in a Polish hospital (a literal case of a documentary that really got under its subject's skin). And what I was left most curious about was why these people consented to their 15 minutes of infamy in the first place? How persuasive must the producers and researchers have been? We viewers may imagine we are flies on the wall; for the participants, the camera crew's intrusion must be more akin to having a rhino on the settee.

Birth of a Salesman (C4), Monday's Cutting Edge film, as much as admitted this: during one of those sequences when someone is being interviewed as he drives, the speaker was concentrating so hard on talking that he pranged his car mid-sentence.

Most documentaries wouldn't have used the scene. But this one, about a group of novice "demonstrators" being taught how to flog pounds 800-worth of duster to their family and friends, threw in every quirky device it could lay its hands on, whether it was employing an oddly irrelevant motif of bottles on a conveyor belt, naming its protagonists with the kind of caption box that more usually contains the words, "Meanwhile, in another part of Gotham City ... ", or shooting the classroom sessions in black and white, with a decidedly restless camera (a technique once used to make a drama series seem more like a documentary, and now used to make a documentary seem more like NYPD Blue). This fly wasn't on the wall, it was very nearly disappearing up the instructor's nostrils.

As well as making the programme exhausting to watch, this approach aroused suspicions about how much content there was beneath the style: about how different a Vorwerk Complete Homecare System was from a Hoover. "Use superlatives! Superlatives generate enthusiasm," commanded the trainees' young teacher, a character straight out of Glengarry Glen Ross. The director took his words to heart.

The resulting film confirmed that would-be salespeople really do act like Steve Coogan's Gareth Cheeseman character, but it didn't show us much more that we hadn't already seen elsewhere - Channel 4's 1994 documentary about Ann Summers saleswomen springs to mind. There was certainly nothing that could be called cutting edge - but maybe we saw enough of those on Cut-Price Package (ITV), Tuesday's Network First programme about plastic surgery. Not a great pun? No, well, you should have heard the ones that Lesley Joseph, the narrator, was trotting out: if it wasn't "mammary make- overs", it was cheap-rate surgeons "cutting corners".

Eight women had flown to Poland for a trip arranged by Alina, a well- meaning native, because cosmetic surgery in eastern Europe costs one- fifth of the UK price. None of these women appeared in much need of cutting and pasting, but they all had their reasons. One of them, explained Alina, was a singer, and had to look her best for her public. Cut (if you'll pardon the term) to the local pub, where the "singer" was belting out another hit on Karaoke Night.

Those poor women. As if it weren't indignity enough to be captured for posterity having your breasts folded back so yoghurt pots could be stuffed underneath, you then have to put up with Dorien from Birds of a Feather nudging and winking about "new boobs" in a chummy, mischievous voice- over which gave away how much of a terrific wheeze she thought the whole thing was.

Still, if the jolly, women's-magazine tone missed the mark, it was resourceful of the director to give it a try. Faced with the sight of an asthmatic, diabetic young girl going abroad to have her fat sucked through a homecare cleaning system at a bargain-basement price, few directors could have resisted turning the situation into a judgemental, jingoistic horror story. In fact, four out of eight of the women were happy with the results of their operations - which isn't a bad proportion for plastic surgery in any country.

Back in Blighty, there was no question why the participants in The Museum (BBC2), Wednesday's supremely entertaining Modern Times film, allowed themselves to be on television: they were born to it. The staff at the Victoria and Albert were proud to work in "a nature reserve for eccentrics", "the extension of a west- London loony bin", and their colourful personalities made for the pilot of a promising new sitcom. The premise was that the museum was there to give its workers somewhere to hang around all day. Preserving an objet d'art is all very well, but it's just as important to preserve the great variety of ways which people can react and relate to it.

For one curator, the museum simply allowed him to don the age-old uniform of the eccentric, in the belief that it is quaint and individual to sport a waxed moustache and a watch-chain. The prim, acid-tongued women at the entrance-hall desk took the role of Greek chorus, observing slyly that some of the people attending their new exhibition "seem to be wearing William Morris wallpaper". And a security man could confess to being driven almost insane by the craftsmanship of a 19th-century model staircase - but he knew his place all the same: "You can't argue with the Trustees. They're the boss, like the boss indoors is the wife."

For this was not a comedy without bite. It developed into an ironic indictment of the English class system, which remains firmly in place, even while modern times erode the certainties that once went along with it. As the V&A invited the Royals to peruse the designs of William Morris, who believed that "art would be the means by which the common man would be liberated", the common man was at his post in the security office, glimpsing a Highness or two as they passed his window.

The main event of the documentary was the introduction of compulsory admission fees, a move precipitated by Government cutbacks. But the viewer felt less for the public who would be deprived of entry (as they were, by and large, deprived of appearing in this film), than for the people threatened by redundancy and by the outside world. "Sometimes I feel as though I've become a museum object," said one sweetly ineffectual chap, who started work at the V&A in 1966 because he got off at the wrong Underground stop for his job at Harrods. "I occasionally look under my foot to see if there's a museum number on it, in black ink." In future, visitors may be paying as much attention to the staff as to the exhibits - if it's possible to tell which are which.

The first time I was due to review a week of television, a colleague sympathised: "Oh ... that means you'll have to watch a lot of documentaries." Just to give drama a look-in, then, Prime Suspect 5 (ITV, Sunday/Monday) deserves the praise that has already been heaped on its four predecessors, but there's no need to add any more. I'm sure we've all read enough about Helen Mirren's brilliance. Instead, a word for the supporting cast, who were new to this latest instalment, but came to it fully formed. Here's to DI Devanney, Henry, and especially David O'Hara as Jerry Rankine, a laconic, frowning bin-bag of a man, who was always one slouch ahead of his boss. If this is indeed Mirren's last adventure as DS Jane Tennison, the makers should think about doing a Taggart, and continuing the series without its principal character. I look forward to Prime Suspect 6: The Next Generation.

David Aaronovitch returns next week.