Plague Over England, Duchess Theatre, London; Saturday Night, Jermyn Street Theatre, London

Sympathy for that talented devil

Part fact, part fiction, Nicholas de Jongh's play attempts to portray, in two hours, the changes in homosexual life and public attitudes over 20 years. But, if Plague Over England, by the theatre critic of the Evening Standard, is occasionally heavy-handed, Tamara Harvey's bouncy production and witty performances, holds our interest. As Binkie Beaumont, the gay West End producer who wanted to throw John Gielgud to the wolves after his arrest in a public lavatory, Simon Dutton is all suave menace.

Though too brusque and good-sporty as the hostess of a gay club, Celia Imrie is an engaging combination of world-weariness and warmth as Gielgud's then co-star in the West End, Sybil Thorndike. Best of all is Michael Feast, who twice acted with Gielgud. Supremely sensitive, he is not only convincing as the second-greatest actor of the 20th century but is at times physically painful, making us feel the terror of Gielgud's arrest with sickening force. Unfortunately, Feast is given a frustratingly narrow range in a play which reduces him to a footnote in gay history. His character is given no activity or trait that does not relate directly to the plot, and too often strains our sympathy with his vagueness and foolishness. The leap from the Fifties to the Seventies is hard to follow (and not helped by Imrie's hostess wearing the same glittery number in both).

The thicket of unexplained references can be confusing as well. I think it far from certain that Gielgud's arrest made him into a braver and less conventional actor (age and talent have a way of doing that). And the presence of not one but two fictional gay young couples (one of the men a copper who, at the blink of an eye, becomes a Joe Orton pin-up) awkwardly changes the mood from sharp to sentimental, from history to fantasy.

Sweet, tuneful, modest, Stephen Sondheim – can you spot the odd one out? Yet Sondheim's first musical, of 1954, is just such a work. It was also unlucky – halfway through auditions, the producer dropped dead, and the show waited until 2000 for its New York premiere. Sondheim had to settle for being a lyricist until 1962 and A Funny Thing Happened on the Way to the Forum, so Saturday Night is a rare chance to hear what the young composer could do.

The show is set in 1929, when Gene, a Brooklyn boy with a lowly job on Wall Street, wants to get rich quick playing the stock market. He and Helen, another dreamer, try to deceive themselves and each other before they come round to the joys of low-rent love.

Screenwriter Julius J Epstein's story is so light as to be so-what, but the songs have a winning charm, especially "I Remember That"and "Isn't It". Sondheim found his voice early – you can tell whose show this is from the first four notes and the first lyric, a love-hungry plaint by a male chorus. There are many amusing intimations of later work: "Exhibit A", a lecture on seduction makes one think of "Now" in A Little Night Music. But, though there's an excess of cleverness, there's a lack of conviction; not surprisingly, the jazzy Sondheim cheers rather softly for the simple life. And, with a hero who exploits his friends, and a score that never summons up the Twenties, sympathy and suspension of disbelief are hard to come by.

These problems are emphasised by Tom Littler's production. In the past generation, British musicals have been getting over their embarrassment at characters who burst into song, but Littler, in the old style, confuses modesty with self-effacement. His cast's acting and singing are tentative, with two actresses (Joanna Hollister and Charlie Cameron) outshining the heroine. If only everyone made the kind of effort that produced Victoria Johnstone's delicious frocks! It's a feast for Sondheim fans, but not everyone will love Saturday Night.

To 16 May www.ambassadortickets.com , To 14 March. www.jermynstreettheatre.co.uk . 020-7287 2875