Additionally, is it normally the check valve that foobar's, would it be dislocated in the pipe'per'ridge.
Another solution... book your car into VW for a winter check, iirc it's about £20, you get a free oil or winter pack, both worth about £15... they will give the car a full going over, scanning it etc. etc. and report back with any faults and parts (with part numbers and blown diagrams) of problem areas... which is normally worth about £60 on its own.