Ok, so I've found that the MAF's ground is not so great. I've measured a consistent 0.8V on it which is thus telling the ECU that there's more air than there really is entering the engine because the MAF uses it's GND as a 0V reference i.e. MAF sends 1.2V, ECU sees 1.2V+0.8V = 2.0V

Sure, I can just give it a new ground, but are there common grounding points that often fail/corrode on the GC8s? I couldn't see any obvious ones that I would have disturbed when I took the inlet manifold off but I'm all ears! I think I saw a wiring diagram on this forum so I'll have a look at that for ground points tonight