Plays don’t come any more topical than Speechless, now on a late-night run at the Etcetera Theatre in Camden. The scene is set: the Prime Minister, only a year out from an election triumph, is in deep trouble, pressured by the media, and many of his colleagues, including a restive Chancellor, to leave his office. Lest any particularly dense member of the audience should miss the parallels, the walls of the office in which all of the action is set is decorated with cuttings from the latest scandals.

Speechless begins with the government’s star speechwriter, Myra MacDonald (Suzanne Harbison), being given the job of drafting a speech for a desperate, surprise move, a snap election, by her putative boss, the slimy Charles Bannerman (Paul Cassidy).

Unlike his real-life (now shipped off to Brussels) parallel, Bannerman is definitely heterosexual, in a shirt-unbuttoned-to-show-chest-hair way. But it is not his sleaziness, or the fact that his pass is an obvious attempt to respond a taunt that his wife is sleeping with the Chancellor, that causes Myra to reject his advances. Any classy, high-powered woman like her would. But, it soon emerges, she has more reason than that.

It is a promising scenario, and the play benefits from Harbison’s strong stage presence. She powerfully conveys the depths of her disillusionment with the souring of the Labour dream, and conveys the further reason for her reluctance to respond to her boss’s clumsy pass — that she has been sleeping with the PM — in a subtle, classy way, long before the script reveals the fact.

Cassidy too is solid enough. If the one-dimensional character or Bannerman gives him little to work with, he does a nice job of making the romantic Albanian coffee-boy Giorgio sympathetic, and an interesting job with his role as another interloper in the office, Myra’s newly acquired bit-on-the-side, and drug dealer, Sean.

The problems arrive, however, with the script. Publicity for the show proclaims it to be “as sharp as Yes, Prime Minister. Ah, no. Not even close. There is some humour in Robbie Moffat’s effort, but it is situational rather than verbal. There are precious few laugh-out-loud lines, and only a few that produce a mild buzz on the funny bone. Bannerman’s line about the impossibility of fixing a British general election vote – “I know, I’ve done a feasibility study” – was one of the few to show any effort at repartee.

And Myra is given a curious trait – she has only two forms of speech, either quoting from famous speeches, or sounding as though she is, and foul-mouthed, uninventive invective. The idea of a speechwriter so taken over by her job that she can only manage Mandela- or Clinton-speak is an interesting one, but it doesn’t belong in this play, producing long, slow passages that reveal lots about the playwright’s political views and do nothing at all for the action.

One good twist in the plot is notable, but the final explosive event – which draws on (in political terms) ancient history – fails utterly to provide a satisfactory conclusion.

There are further minor irritating aspects of the production: the lighting is all over the place, the movement around the stage feels under-rehearsed, and Moffat, who also directs, really should stop using free copies of the Camden New Journal as stand-ins for the national first editions (very distracting!)

Speechless continues at the Etcetera Theatre until May 21, at 9.30pm, (Sundays 8.30). Tickets are £10/£8. Box office 0207-482 4857