Last Night's TV: All Roads Lead Home/BBC2Fresh Meat/Channel 4

Some television programmes just make you shake your head violently, as if you've got a bee stuck in your ear. Take All Roads Lead Home, for example, a three-part series about natural navigation.

Come again? A three-part series about finding your way around using only the sun and the side of oak trees as navigational aids? Three whole hour-long episodes about how churches run from east to west and some lichens only grow on the north side of gate posts? Why, in the name of God? "This is a programme that rips up the map," they boast in the opening credits. And, yet again, why? Why would you do that unless you were mad? Frankly, it was bad enough just reading the billing in the Radio Times. "Just one wrong turn could result in disaster," it declared excitedly, broadening the definition of disaster to include mild embarrassment. And, by the way, if they really believe All Roads Lead Home, they're screwed before they even set off.

It gets worse once you start watching, with a slightly dizzying speed. Sue Perkins, Stephen Mangan and Alison Steadman present, sort of jointly, as if each one of them hoped that the other two might end up taking most of the blame. The idea is that they're tutored in natural navigation techniques by Tristan Gooley – "a real-life action man" – and then are sent off to walk from A to B using only their acquired knowledge of the book of the earth, and a highly detailed point-by-point description of the route. I suspect that they initially make quite a lot of use of those inscrutable features of the landscape – signposts – but after a while they generally find themselves in a field, gesturing at the sky and trees and saying things like, "That's east, isn't it?" "No, that must be south." "Well, if that's south, then that must be north-west." "No, that's north-west! This is south-east." It is as much fun to watch as it is to read about it.

There is some unintentional comedy, besides the low-key banter between Mangan and Perkins. It's funny, for example, to see how much effort they have to put into making the navigation look tricky even when they're literally on a beaten path direct to their destination. They bend over to scrutinise animal droppings for evidence of the direction of sunlight, ostentatiously ignoring the giant orb of incandescent gas blazing over their shoulders. In one case, their target was on top of a hill, not exactly a navigational challenge even on an overcast day. "There aren't many places where you can just stand and see 360 degrees right round," said Steadman wonderingly when they reached the summit, gamely trying to talk up the view. Actually, Alison, there are millions of them. You can do it by standing on a kitchen chair if you're not too fussed about how far you can see.

Then again, I have a suspicion that Steadman was just being a trouper throughout, doing her best to whip the thing into life. "Gosh! It just gets more interesting by the minute!" she gasped a bit later, after a local botanist had showed her "an unusual and directional shrub". After which, it was time for a sequence in which we literally watched lichen grow and another fascinating discussion about compass bearings ("I think this is east" "And that's west?" "Then that must be south. Is it south?" "No, I think it's west"). At the beginning, Tristan Gooley offered this advice: "The best way," he said, "is to begin with a really simple question, 'Which way am I looking?'" Any way as long as it's not towards the television, I'd suggest.

Fresh Meat, now that it's moved on to writers other than Sam Bain and Jesse Armstrong, occasionally lapses into Yoof callow, a form of comic writing that privileges the brutally scatological over anything more nuanced. There was a nasty exchange in last night's episode, which stuck out like a... well, let's say a besmirched appendage. But there's also much better stuff, whether it was the silly comedy of Howard talking through his all-you-can-eat buffet tactics (which include a concealed bin-liner) or a lovely line from Tony Gardner's Professor Shales, who at first looked like a cocky sexual predator but has now revealed himself to be overshadowed by his more successful wife: "Jean thinks I'm Plath," he whined to Oregon as he attempted to seduce her. "But I'm not Plath, I'm Hughes." Cut the self-conscious filth from some comedies and there wouldn't be a lot left. Cut it from Fresh Meat and you'd be left with the best stuff.