Ok so good news I hope.
Techstream has found a short/open circuit in the injection system and I have an error code for an injector too.
I used the active test to individually cut fuel to the cylinders. And sure enough there was no change in the idling when I did cylinder 3. So this poses a few new questions...
firstly, if you take away all the stuff about thinking it was the turbo, could this explain my symptoms? (Loss of power, rough/jerky idle, slight knocking noise under the bonnet)
And secondly, is it best to get this injector repaired or buy new? I can code in the new injector with Techstream.