The Hon Elizabeth Montagu was the dashingly Mitfordesque aristocrat and adventuress who worked for British intelligence in continental Europe during the second world war, after which she made a glamorous, if little credited career as a screenwriter and fixer in British cinema for Alexander Korda before achieving micro-fame as the publicly loyal ally and confidante of her brother Lord Montagu of Beaulieu, when he was convicted in 1954 on a gay-sex charge.

This extraordinary life probably needed a solidly presented non-fiction film; what we have here is an odd and lumpily constructed drama-doc with daytime-telly production values, featuring some very hammy acting and direction. Diana Rigg narrates from Montagu’s own memoirs, and there are sporadic on-camera interviews with friends and family, and black-and-white portraits of the real-life principals flashed up on screen.

But then there is the laborious drama, with its sometimes toe-curling dialogue. Everyone is doing their best, and Dorothea Myer-Bennett is actually well cast as Elizabeth, but too often the cast looks like a provincial rep company in a remake of the ITV French Resistance drama Wish Me Luck.