How weird it is to discover the eternally Left-wing playwright, Howard Brenton, has sketched a sympathetic, spell-binding portrait of Harold Macmillan, that patrician Conservative prime minister.

A beguiling, bespectacled Jeremy Irons, adorned in a luxuriant moustache and inspired by Brenton’s script, does not offer the Macmillan impersonation that Peter Cook made famous. Instead he concentrates upon conveying what Brenton reveals of Macmillan’s inner life: the guilt about surviving the first war and the man’s neurotic sense of inferiority. These feelings were perhaps heightened by his adulterous wife, Dorothy, a duke’s daughter, but were screened by this witty, showman premier’s pose of unflappability and self-assurance.

Bringing himself into sympathetic realignment with Macmillan, Brenton uses this politician’s brilliant career to raise fascinating questions about Britain’s aspirations in the grim first half of the 20th century. He implies that British notions about our world role today, with intervention in Iraq and Afghanistan, are not that different from Macmillan’s half a century ago. Accordingly I found myself stirred and moved by Never So Good, with its deft and humorous encapsulation of the future premier’s early life. Brenton refracts the events through the prism of the old man’s memory.

It is his poignant, imaginative conceit that young Macmillan (Pip Carter) in his soldier’s uniform haunts the action and the older man like a reproachful ghost — “I’m dead in you” he disquietingly says — mocking and belittling. His mother (Anna Carteret) becomes, in defiance of Macmillan’s own autobiography, contemptuous of her pseudo-socialist, Thirties son. Brenton sentimentalises the politician’s rocky marriage and the strange, enduring affair between Anna Chancellor’s far too glamorous Dorothy and Robert Glenister’s bisexual Bob Boothby, who hungered for power and gaily mixed with criminals.

Howard Davies’s expressionistic production works like a dream and, in a sense is one. Vicki Mortimer’s beautiful, bleak stage is bare but for a row of huge, high doors ranged all along one wall, which open to reveal stacked, mobile shelves of documents. Mark Henderson’s superb lighting offers dreamy shafts of illumination — on figures in St James’s Park, on tangoing dancers beneath chandeliers at the Ritz who scatter to the warning cry of “gas”. These warnings give way to explosions in Hyde Park and, seamlessly, to horrors in the trenches. How can you not admire a man wounded five times in war, who, when shot, rolled into a foxhole, injected himself with morphine and read Aeschylus till help came in eight hours?

Macmillan’s bravery in two world wars, and his rebellious support for Churchill against appeasing Chamberlain, are represented as battles to save civilisation. At Suez, though, when Anthony Calf ’s sick, obsessed Anthony Eden conspires to take the Canal, Macmillan, both a Judas and Brutus, accepts Britain’s role as an invasive imperial power. His subsequent struggle to maintain Britain’s independent nuclear deterrent, to cut the paternalistic ties with Africa and join the Common Market, suggests how conflicted Britain has fought to remain a world leader in changed times. Never So Good thrillingly dramatises an unsolved problem about our role in the world.