How old's the car - one of the things that sometimes can cause a pong if it's not been done for years is pollen filters - had one of htose on the mk4 stink it up. Also, any condensation?
CkB - its a 55 plate - sounds like its a common problem although its the first car that it has happened to me
Doesn't suffer from condensation problems but the car is 'garaged' every night - there's a pool of water on the garage floor most mornings (from the air-con?)
wonder if its something to do with air (lack of) circulating around the car? [
Also will try running the engine with the heater on full-bore for 10/15 mins later