For a start, they make all manner of noise. As they try to take "Heaven by Storm" - Shane is pursuing Dave across the universe for 20 cents towards the costs of his pet cricket's funeral - they conjure up the sounds of everything from Alka Seltzer in a glass of water to those machines that go beep in a space-rocket merely by using a microphone and an over-active larynx.
In one sweat-inducing routine, Collins, in a red vest and braces, mimes an increasingly frenzied, gun-toting Travis Bickle from Taxi Driver while Dundas, in, er, a red vest and braces, stands at the back providing the sound effects - including a super-charged "You talkin' to me?" Imagine Robert De Niro animated by Hanna Barbera, and you get some idea of the cartoon mania of the scene.
But it's as much about exercising the body as the voicebox. They are not averse to running up the wall if they think there might be a laugh in it (which there is). An eye-watering skit in a multi-gym - set, inevitably, to "Eye of the Tiger" - culminates in them frantically trying to outsprint each other on a running-machine. They must be as fit as several fiddles.
They can also flex bon mots, artfully de-constructing the action as they go along. To end one scene, Dundas says he has to go to a meeting with Andrew. "Who's Andrew?" asks Collins. "He's just a sub-plot I've been working on," comes back the reply.
There is a certain breathlessness to the whole affair - like a child trying to impress his parents by running round in a circle till he feels sick. And the plot - throwing in pastiches of Terminator, Star Wars, and that mimed Guinness ad apparently at random - is about as comprehensible as an episode of The Prisoner. And they might want to consider seeing someone about their gun fetish.
But the Umbilical Brothers offer an appealing mixture of deftness and daftness, and a verbal and physical dexterity you wouldn't usually associate with the country that gave the world Sir Les Patterson.
n To 16 March. Booking: 0171-836 2132