Thanks for the replies! I seem to have fixed the issue (touch wood). I've been struggling with an intermittent stalling fault (and then no start until cycling the ignition) and a high RPM engine hesitation/miss which was stopping me from even reaching redline under boost. I was also suspecting an ECU fault, so I pulled it out again and checked it out, the capacitors had been replaced (albeit quite an ugly job was done with the cleanup and repair of the PCB tracks) so I was almost certain this was the issue.

Turns out my CAS was faulty, this was baked years ago from being too close to the single turbo manifold with no heat shields (I fitted heat shields after a couple of days, evidently the damage was already done). Replaced the pickup module with a 2nd hand unit and the engine is now running at 100%, funnily enough, the LCDBC also seems to be working as it should.

I haven't had a chance to scope the faulty CAS module yet, I guess it had a dirty signal output which got worse as the revs and load increased until it threw the ECU into a fit.