Early on in this fitful alternative history of VE Day, the future Queen Elizabeth encounters a piece of gossip concerning Gregory Peck. We should give the film-makers some credit for this oblique nod to an obvious inspiration. Like Roman Holiday, one of Peck's best films, A Royal Night Out sends a princess – two princesses, in fact – out into the world to meet rough reality and very nearly fall in love. Unfortunately, the picture is dragged down by shallow staging, hugely broad characterisation and some unevenly matched struggles between actor and accent.

A Royal Night Out imagines that, on the last night of the war in Europe, sensible Princess Elizabeth (Sarah Gadon) and dim-witted Princess Margaret (Bel Powley) escape a party at the Ritz and launch themselves into West End lowlife. The two get separated and Elizabeth finds herself teaming up with a disillusioned airman (Jack Reynor) to track down her errant sister. (Weirdly, the core plot is the same as that of Night of Triumph, a recent, apparently unrelated novel by Guardian film critic Peter Bradshaw.)

For the most part, the actors "keep calm and carry on" with some dignity. Reynor convinces as a working-class Londoner. Rupert Everett seems to have decided to play the Duke of Edinburgh rather than, as requested, King George VI, but the performance works well anyway. Emily Watson is a more convincing Queen Elizabeth than Helena Bonham Carter was in The King's Speech.

The two women leads are, however, presented with near-impossible tasks. The respectful (one might almost say hagiographic) version of Elizabeth is so dull one finds oneself yearning for a bit of Spitting Image.

The party-time take of Margaret is less polite – if not necessarily less accurate – but broadens the character to indigestible 1970s sitcom levels.

Thank heavens for the older pros. There is an entire film to be drawn around Roger Allam’s portrayal of a spiv who, though handy with his fists, loves the royal family unreservedly. Such men still exist.