Colin Clark’s memoir, The Prince, the Showgirl and Me, contains more than a touch of sad wish-fulfilment about how in 1956, as a lowly third assistant director, he struck up a friendship with Marilyn Monroe when she was in Britain to shoot The Prince and the Showgirl opposite – and under the direction of – Laurence Olivier. My Week with Marilyn takes Clark’s memories at face value and posits that he (Eddie Redmayne) and Monroe (Michelle Williams) snatched kisses, slept in a bed together (but didn’t inhale), swam naked in the Thames and gigglingly talked their way into Windsor Castle on the back of Clark’s connections (his father was art historian Kenneth Clark).

Simon Curtis’s pretty but insubstantial film also pitches a theatrical and impatient Olivier (Kenneth Branagh) against a highly strung and strung-out Monroe, so that My Week with Marilyn is partly a flighty reconstruction of an odd moment in film history and partly a Roman Holiday-style romance.

The saving grace is Williams. Various actors, from Toby Jones as Monroe’s press agent to Zoë Wanamaker as her Method coach, flap around her in underwritten parts, but Williams is convincing both as a troubled star and a luminous on-and-off-screen presence.

My Week with Marilyn is a minor movie about a minor episode in the life of a major star, but it has enough sense of its own high camp not to take itself too seriously. It’s pedestrian in most ways, but Williams – with help from friends in the lighting, camera, hair and make-up departments – lends it a touch of magic amid the nonsense.