While the pay off is perhaps obvious. The bumper -the line after the joke that's there to get the big laugh - works really well. Nice performances too.
Then there's this one. I thought this worth commenting on for the quality of the writing and the fact that they've made the assumption that the viewer will get what's going on without any heavy explanation - we're made to discover at the same time as the guy at the bar does. (That said, the strapline so doesn't work.)