Check all the pipes and especially, check all the little vacuum pipes which emanate from the box behind the drivers side headlamp. My guess is that it's one of those, or the actual N75 valve that's at fault. This link, even though it's for T4 Transporters, gives some good info and stuff to check:
Let me know how you get on - I'd not have much time on Sunday to do anything other than a scan, otherwise I'd have said come over anyway, but it sounds like the root cause has been found, now it's just a case of tracking down the culprit, which could take 20 mins, or it could take a few hours.
Johnny it’s the black box on the drivers side inner wing. I pointed it out to you when I nicked the ecu out of the breaker a few months. Suggest checking the pipes first and then if nothing there swap the black box front the breaker to your lads car
On the earlier cars the black box was the 3 valve on the bulkhead but was cleaned up on the latter cars with a single unit on the inner wing
John, I've put an arrow on your photo showing the location of the vacuum valve control box. Check all the pipes exiting there - I had a limp home situation on Tank before Xmas and a quick fettle with the tubes (I could feel them but not see them behind the engine), sorted things.
Ok mlads, managed to swap over the black box thingy, started the car and still same, ticks over nice but when reaching 2000 revs it goes a bit loopy, engine suddenly revs high then falls and I can tell sumats not right.
Question is now, will the ecu still be in kind of limp mode? so will need clearing?