The throttle position switches don't click on the turbo M2.7 or M2.8 because they are potentionmeters.
Out of the cam cover at the back are two pipes.
Small one goes to the throttle body via a one way valve.
Large one connects to the turbo at the front.
It would appear that the one way valve is made out of bio degradable material ! They cost about £10 ? so I used a very strong windscreen washer one way valve as I thought the new valve would too be usless.
B'locks, wish I'd spotted this post earlier - picked up on this problem on my car and someone elses recently.
If you listen carefully, you can actually hear the air whistling out under high boost (if the valve breaks at the right end)
As Andy says, just put any type of one way valve in here - so it can vent to the inlet manifold, but not blow boost pressure into the cam cover.
The heat in there really seems to go for these little chaps.
As Andy says, dead easy to fit, just a bit fiddly and don't overtighten the jubilee clips and crack the thing.