I'm guessing your car is an 05 or 06 plate. Many suffered exactly the same problem including mine. Curiously, VW replaced the rear wiper motors under warranty and that cured it. Not sure if VW would pay (assuming you're out of warranty). Mine used to only very occasionally do it and I could have lived with it if it was an expensive fix.
You might be able to get a motor off a breakers or even second hand. I think some on here have removed the rear wiper for looks and may have a motor to flog?