Thanks very much - all useful stuff.
The other purge valve is under the big solid Turbo air pipe that runs past the throttle body. I took this pipe off to thoroughly check the flexible parts, and underneath it hey presto was the other check valve. It's the same design as the more visible one on top of the radiator fan cowling. On mine the second purge valve was working OK so I put it back.
As mentioned, I doubt that these purge valves are going to have much bearing on the turbo boost issue, but I just think that while we're going through the whole vacuum system to make sure it's sound, then it's worth changing these too if they're faulty.
I got a supply of vac hose this morning, so as soon as I have time I'll replace the remaining crusty vacuum lines, and check a few other things that have been mentioned above.
Cheers
