I compared the ECU reading from the manifold collector with my LM-2 reading from the tailpipe today, not a million miles off each other and my readings are much better than they were previously. I'm running in a new engine, I've smelt the oil and I don't smell fuel from it, hopefully all is ok.
I was aware of this issue, I had a blob of weld added to one end of each clamp to help them reach, when I refitted the manifolds today I observed that the inlet was actually thicker than the exhaust, for some reason I had fitted the clamps the other way round last time. Hopefully the seal is better this time round.
Thanks Tom, unfortunately they aren't the easiest to buy from online, I'm not going through this nonsense of someone calling me to take card details. I've just fitted a thicker gasket from Revingtons this afternoon, the idle has improved and the AFR, although not constant, is certainly not quite as wild as it was before. I will check the exhaust joints over the weekend and see if there's any leakage through there.