‘You can’t start with a pause’ says Birdboot to Moon at the top of Tom Stoppard’s The Real Inspector Hound, but Mike Poulton and director Lucy Bailey disprove this most successfully as Paul Keating lies before an unlit hissing gas fire in the opening scene of Kenny Morgan. It parallels Terrence Rattigan’s The Deep Blue Sea where Hester Collyer attempts suicide when discarded by her virile, alcoholic younger lover.

As Rattigan may have appropriated real-life experience for his play, Poulton re-imagines the situations in a seedy Camden Town boarding house where distressed Kenny is torn between a glamorous but secretive demi-monde as the urbane playwright’s lover – a secret necessarily kept in the years they were intimate through and just after the war – and a no less dangerous affair with angrily bisexual and under-aged partner Alec played bravely to the point of extreme unlikeability by Pierro Niel-Mee.

Lengthy exposition and some repetitive scenes mean the central love triangle is less wholly engaging than the ancillary characters who people Robert Innes Hopkins’ evocative gas-piped tenement, beneath the light spill from Jack Knowles’s brutal North London window. Each in his own way is fascinating: landlady Marlene Sidaway sounds close to Doris Hare’s ‘Mum’ from ‘On The Buses’ as a perfect distillation of old-fashioned East Endery before that was cheapened by Albert Square, and her slyly prejudiced asides are thrown away with just the right amount of guile.

George Irving plays Mr Ritter with immaculate detail – a Viennese émigré struck off the medical register for reasons never explained and a direct parallel to Mr Miller in The Deep Blue Sea who befriends Hester, and who provides the conscience of the piece.

But I was most closely drawn to the simplest character, Matthew Bulgo’s carefully, realistically crafted bashful Welsh neighbour, a clerk in the Admiralty who once yearned for the ocean and whose third act paean to ‘ordinariness’ is an anthem for the common man, and his advice to the vain playwright to ‘spend more time sitting in the audience’ more apposite than reams of criticism.

This is a great piece of work for Paul Keating: he makes Kenny both deeply pitiable and actually undesirable, maintaining a forced peevishness even in the face of extreme kindness from lovers and neighbours, and measuring his neurosis across the evening with admirable control. The climactic scene with Rattigan comes later than you’d wish and is followed by an equally devastating scene with Alec, but both are worth the wait.

Simon Dutton’s Terrence is a convincing grandee and you believe in his earnest protective morality, but perhaps too handsome and suave: I recall the older Rattigan as quite an unpleasant man, black cigarette holder permanently clamped between ochre teeth rather like Burgess Meredith as the Penguin in Batman – whereas Dutton makes him a sexily smooth Rex Harrison.

Very well worth a visit. The only shame is that the National Theatre didn’t commission this piece to run alongside its upcoming revival of The Deep Blue Sea with Helen McRory.

But then again, Rufus Norris missing a trick – who’d have expected that?

Kenny Morgan continues at the Arcola Theatre until June 18.