Sarah Helm’s first play is an imperfect, bumpy construct, but absolutely compelling in its semi-autobiographical picture of a middle-class domestic household shaken, both physically and ideologically, by the Iraq War.

Helm is a first-rate journalist who lived with (and is now married to) Jonathan Powell, Tony Blair’s chief of staff in Downing Street, as the war looms. Her stage surrogates are Laura (Maxine Peake) and Nick (Lloyd Owen): their large double-bed in Stockwell is strewn with red cabinet boxes and telephone interception equipment.

No, this is 2003, so it’s not yet phone-hacking time: Nick just listens in on all Tony’s conversations with Kofi Annan, George Bush, Rupert Murdoch and the CIA. So does Laura. And their Polish au pair, Marisia (Anna Koval), is pretty clued up, too.

The weakness of the play is that Laura and Nick are unshakably devoted to each other despite their political differences. Dramatically speaking, something more catastrophic than protestors on the street outside and a faulty alarm system is called for.

And in the second act, inside Downing Street, the play sets off in an entirely different direction as the Weapons of Mass Destruction scare is traced to a hoax in Germany and there are side-swipes at Andrew Gilligan’s exposure of David Kelly (who later killed himself) and the malign, distant presence of Donald Rumsfeld.

For all its faults, and lumpy dialogue, Edward Hall’s production is hugely enjoyable. Peake and Owen are immensely attractive actors and Patrick Baladi’s Tony is a beautiful portrait of hesitancy, deference and vanity, subtler than Michael Sheen’s for being less physically insistent. We learn that Blair only buys shoes at Church’s, deflects calls if watching The Simpsons, reads cuttings (not newspapers) and play tennis only with his coach.

The second act is bolstered by the presence of Michael Simkins as a blustery, casuistic head of MI6 and Colin Stinton as a whisky-glugging director of the CIA. The overwhelming impression is of the blind leading the blind as Blair stumbles into the war for all the wrong reasons; Nick protests to Laura that “we were right to be wrong” about Iraq and that telling the truth is never as easy as it sounds.

Rich in such telling detail, as well as insider information, the play is expansively laid out in Francis O’Connor’s splendid, free-standing design of the Stockwell bedroom and kitchen, and the interior of Number Ten. In this domestic and political cocoon, squabbling over the war becomes more significant than its effect on thousands of unseen innocents, the play’s bleakest irony of all.