Last Night's Viewing: Dirk Gently, BBC4 The Bleak Old Shop of Stuff, BBC2 Watson & Oliver, BBC2

How much plot can a comedy sustain before it starts to get stodgy? Obviously, the answer to this question is going to vary (last night offered two comedies that made that very clear), but to my mind the ratio between narrative scaffolding and comic liberty needs to be pretty tight. Let the storytelling get out of hand and, unless the plot itself is a wellspring of hilarity, you may be left with the sense that there's not quite enough sauce to cover the meat. You prepared for a poussin and suddenly there's a 20lb turkey on the table and it's dry going. Dirk Gently is a good case in point. Nothing wrong with the sauce, which includes two very fine comic actors in Stephen Mangan and Darren Boyd and some nice moments from Howard Overman's script. It's just that those qualities in the end spread a little too thinly over a nonsensical thriller plot.

It's supposed to be nonsensical, of course – Dirk's belief that "everything is interconnected" pretty much necessitating a chain of wildly improbable coincidences and consequences. But since anything can happen you don't very much care about anything that does, and Dirk's metaphysical musings about "Zen navigation" and the complexity of the world begin to get repetitious quite quickly. There were laughs, including a nice reveal when Mangan opened a Valentine's card in the middle of a complacent speech about his powers of attraction to find that the inscription inside read "I hate you, you're a pig". But they were far too widely spaced in a script that could have done with a lot more editing. Scorning someone's belief in astrology, Dirk asked him whether he really believed that planets "billions of light years" away could affect human destiny. Millions of miles would cover it, Dirk, and yes, you might justly point out that this scientific pedantry is irrelevant. But I probably wouldn't have noticed if he hadn't used the same phrase three times. Or if I'd been laughing enough to distract myself.

The Bleak Old Shop of Stuff is stuffed with plot but gets away with it because that's one of the essential gags. As a spoof of the intricately engineered clockwork of a Dickens novel, full of sudden revelations and shock reversals, it could hardly be any other way. And in any case, it always takes care to have a joke on hand to lubricate every narrative turn. So, when Conceptiva Secret-Past and her daughter Victoria use Primly Tightclench's deportment volumes to bludgeon their way past the baddies you get a quick close-up of the titles they've picked: "How to Hurt a Large Man" and "Self Defence for Girls". I wasn't entirely sure about the first one-off special of Mark Evans's comedy at Christmas, but it's far easier to surrender to its silliness now that it's been sliced up into half-hour portions.

The cast is excellent, with Robert Webb relishing the possibilities for guileless credulity and Tim McInnerny chewing the carpet (in a splendid way) as the dastardly Harmswell Grimstone. At one point last night, he paused in the middle of a triumphant cackle as if something was missing, stroked his upper lip and said pensively: "I really must grow a moustache to twirl." I enjoyed the trial scene a lot too, in which Harmswell arrived understandably confident that he would prevail. The judge was called Harshmore Grimstone and he'd taken advantage of the immemorial right of every Englishman to be tried by a jury of his cousins.

Watching Watson & Oliver, I just find myself thinking how old-fashioned the format is. The awkward-intro routine was getting a bit old when Griff Rhys Jones and Mel Smith did it back in the late Eighties and the comic dynamic seems too obviously indebted to French and Saunders. They are both talented, though – comic actresses as well as comedians. A more up-to-date vehicle would help.