The part number is "027 133 985 A" and costs £66.61 from the dealer. Unfortunately it's back order only, so it could be some time before I get my new one, 10 days at least according to the dealer. 
Just had a call from the dealer, the valve has arrived already! 6 days for a back order part, not bad! 
hope it sorts the problem mate.
my 8v is driving me nuts because of the same issue. was fixed 2 weeks ago with a new flying saucer breather gromet, but last week developed the same problem again! replaced the ISV which made no difference, and checked all vac pipes. only one i can see being an issue is the short overbraided pipe next to the ignition coil associated with the MPG reading on the MFA, as the braided section has worn through.
the flying saucer and the breather pipe attached to it was all lined with mayo too... worrying... although it might just be were its being left for a few days at a time.
