Inside Fat Pig there is a thin, exploitative black comedy struggling to get out. The play masquerades as a study of our narcissistic age of uniformity, in which blogs and Facebooks encourage indecent exposure of the self and it is thought better to be a slim, beautiful air-head rather than a fat, sexually undesirable artist.

Its brilliant author, playwright and film maker Neil LaBute, specialises in shock-tactics and bad taste in good, liberal causes — though not really here in his own elegant production. Having been seriously overweight himself, a condition which LaBute says he tried to alter before realising being plump and miserable inspired him to creativity, you might think he would be well-placed to make himself his own sad, Fat Pig hero. That role, though, is assigned to Ella Smith’s Helen, a seriously over-weight, but appealing librarian with several lines in jocular self-mockery and wearing those Hollywood, clichéitems for plain girls — a pair of spectacles. When Robert Webb’s Tom meets her eating loads of pizza in a restaurant, an easy conversation begins. “If we’re going to start dating,” she humourously ventures. “What!” he exclaims in the horrified tones of a man who finds fat women sexually unappealing. This first reaction is followed by an inexplicable about-turn when Tom suggests they should meet again. So LaBute gives the couple a contrived relationship, instead of making the romance a natural attraction between fat and thin. Helen’s character is left unexplored, the neurotic difficulties that led to her overeating never touched upon.

The play’s mission, though, is to suggest how hard it is for an ordinary man to do the unconventional thing and fall for someone whose fatness outrages a world of slimline youngsters. And as a comedy of character and situation Fat Pig induces waves of laughter.

In the undefined office environment where Tom works, Kris Marshall’s charming, juvenile lay- and- play-about Carter and Joanna Page in the role of Tom’s far more off than on girlfriend, discover the secret romance and launch their retaliatory, comic manoeuvres.

Carter, whose swaggering elegance, arrogant conformism and cultivated eccentricities Marshall forcefully embodies, not only sneaks Helen’s photo onto the net, but warns Tom “people aren’t comfortable with difference — fags, retards, cripples, old people.” To this unpleasant challenge LaBute’s under-characterised Tom makes no riposte at all. He responds with muted indignation to Miss Page’s superb Jeannie, who launches a blisteringly delivered tirade of abuse, appalled that she can be passed over for a fat girl.

No wonder, then, that he virtually hides Miss Smith’s poignantly smitten and desperate Helen at a work’s beach outing and admits, in Webb’s powerful, tear-struck admission of weakness, that he lacks the courage to continue the affair. This abdication strikes a false note: the sex-drive would not so easily be beaten down by the malice of office colleagues. Fat Pig, as with LaBute’s nasty film In The Company Of Men, shows women being mocked and brought down — respectively for fatness and deafness. I find the procedure repellent and my own laughter shameful.