Well, thanks to
@timmus I found out that there was a little green pipe coming from the N75 valve that was not connected to the turbo housing. I saw where it was at the time timmus found it and had a little feel around but since timmus said that it was a job that you could only do from below I did not try too hard to see if I could re-connect it there and then.
My curiosity got the better of me today and I decided that I would have a go from above. Sure enough it was fiddly and with my left arm tightly behind the engine I could just feel the end of the rubber pipe where the thin green pipe should have connected.
I got my right arm in above the engine just below the air intake and could take hold of the little green pipe from there to push it, with my left hand guiding the little green pipe into the rubber pipe. I do have quite small hands and thin arms but even then I was worried that I might be stuck in my A2's engine until help came...
After a bit of pushing and pulling and twisting and turning the little green pipe slid into the rubber pipe and I pushed and pushed until I was sure that it would not come out again.
After a test drive I thought that limp mode had come on on the way out, but on my return, pushing it as hard as I could up a hill, it seemed fine.
When I got back and scanned it there were no error codes.
After the engine cooled off a little I stuck my hand back down behind the engine and could feel all three pipes were still connected. However the rubber pipe sheaths do feel rather frayed.