Time for Bed by David Baddiel

Gabriel Jacoby can't get to sleep. In fact, he can't get anywhere at all, either in his Triumph Dolomite or his life. Everything around him, from his large collection of coffee-machines to his balding Bradford-born flatmate, is breaking down. Not that Gabriel is bothered; he's too busy being in love with his intensely happily married brother's wife. Which is why Gabriel chooses to waste all his time - because he knows that whatever else he might achieve, it won't be happiness. There's no way there, when you're in love with your brother's wife. Unless you remember your brother's wife has a sister...