Neil LaBute’s Fat Pig is a slight play on a weighty theme. You fall in love with someone who looks unusual, in this case very large – although there is nothing unusual about that in America and, increasingly, here – but have trouble going public about it. What to do?

Like all good playwrights, LaBute operates on two levels. The fatness of his sweet-tempered heroine Helen is both a literal expression of her bountiful humanity and a metaphorical signal of the difference in all people, not just handicapped or culturally alien – but temperamentally unique, too.

For it’s clear in the tortured, wittily inflected performance of Peep Show star Robert Webb as Tom that diving into the enveloping “niceness” of Ella Smith’s beautiful, gargantuan Helen is as much a spiritual tactic as a physical one: he wants a new comfort zone after a needling, nagging relationship with the screeching sexpot Jeannie (Gavin and Stacey’s Joanna Page) in the office accounts department.

So, for him, fat really is the new thin and also the refreshingly new fat. Tom meets Helen in a city lunch bar, right across the road from where she works as a librarian, or “printed word specialist.” Back at the office – you don’t know what business the office actually represents – Tom’s colleague Carter, a supercilious smartass who takes satanic glee in the discomfort of others and is played to ghastly perfection by Kris Marshall, tries to find out what is going on, while Jeannie bitches on the sidelines.

After an idyllic period in which Tom and Helen hunker down with popcorn, old movies and a dialogue relationship that cleverly steps around the size issue which is staring them in the face, things come to a head on an office beach outing. Jeannie looks gorgeous in her bikini and Helen, well, impressive in her swathes of swimwear and sarong.

Huge is a growing issue nowadays; over a third of most people are too fat, and it’s bad for them and bad news for their loved ones. LaBute, who directs his own seven-scene play on a stark, revolving design by Christopher Oram, has come up with another brilliant contemporary comedy and incidentally revealed a wonderful, richly voiced new star in 24 year-old Ella Smith, who really does prove that fat can be beautiful as well as a problem.

- Michael Coveney