Last Night's Viewing: Celebrity Bedlam, E4 Olympics 2012, BBC

"Celebrities... don't you love 'em? Doing adverts, appearing on reality TV and above all talking about stuff they know little about." The obvious answer to Lee Kern's question, posed at the beginning of Celebrity Bedlam, is "No". But then I don't think he much cares for celebrities either or he wouldn't have made them the exclusive target of this prank show.

Frankly, you wouldn't have thought such a thing would still be possible, after years of Sacha Baron Cohen's brand of ambush television and years after Chris Morris persuaded various celebs to plead the cause of a zoo elephant that had lodged its trunk inextricably up its own rectum. But then, if the evidence of Celebrity Bedlam is to be believed, these people don't exactly seem to be keeping a beady eye on developments in the modern world.

Whether it is to be believed has to be an open question. After all, they get final cut on the pranks. And to be fair to those involved here, pretty much everyone expressed some doubt about what they were being asked to do. "This is all true, this is?" asked Howard from the Halifax ads, before doing a piece to camera for a children's science programme, about bats with human testicles. "I just can't believe what I'm seeing," said Paddy Doherty (My Big Fat Gypsy Wedding) when shown a cloned human penis in a specimen jar. "Really? Is that true?" queried Ben (from Big Brother series 11) when asked to tell the "true" story of Brian, the drumming monkey, who did session work on The Verve's "Bitter Sweet Symphony".

To be fair to Kerns, though, these momentary bleats of disbelief appeared all too easy to quieten. A professional commitment to the job at hand might have covered some embarrassments. Yes, Ben looked exceptionally stupid doing a piece to camera with carrots and potatoes Sellotaped to his face, but then perfectly respectable broadcasters have done worse things in the name of education. There comes a point, though, when only stupefaction or being in on the joke could explain their numb biddability. Cheryl Baker cheerfully explained that "if you put a cucumber in a pint of beer you can drink as many as you want and it won't register on a police breathalyser" without apparently questioning either the truth or the ethics of this tip. And David Van Day appeared genuinely horrified when taken to a "genetic research laboratory" and introduced to his own clone, a shuffling zombified figure in a David Van Day wig. It seemed some part of him really believed that a liberty had been taken with his DNA. Whether you think it's any kind of triumph to pull the wool over eyes so purblind is another matter altogether.

Since we're talking about presenters "talking about stuff they know little about" may I just say how much I'm enjoying the BBC commentary on the Olympics, a daily cornucopia of hastily assumed authority and wild jingoism. Some of them do actually know what they're talking about, of course, though that's usually not terribly useful because they tend to mutter terse but gnomic remarks that only other experts would understand. Meanwhile, their translator sidekicks simply tell you what you can see for yourself. And what goes missing in between is all the stuff you actually want to know.

The BBC's pre-event tutorials will tell you how many centimetres wide the springboard is (could any information be more useless?), but not why the divers all go for a dip in the Jacuzzi after a dive. Also troubling me at the moment: what's that little trigger widget that clicks out of the way just before an archer releases the arrow? And what the hell's going on with those table-tennis serves? Rather than saying dumb things, couldn't the civilian co-presenters ask some dumb questions on behalf of the rest of us?

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