Had a good afternoon stripping everything back down and checking for possible causes of air leaks.
First issue was a bit of plastic that had got caught on the O ring grooves on the plenum. Had deformed the new o ring slightly but nothing irrecoverable. I discovered why I cannot detach my inlet and injectors completely - it's been hardwired into my LPG ecu. Should have realised that first time around. Next interesting discovery was the air vacuum valve at the back of the right hand bank had broken off - completely. Further investigation revealed this had been broken and then repaired with epoxy. The mix was a little heavy on hardener and it had become brittle. My fiddlings earlier had caused it to separate. I believe this was the root of unmetered air getting into the cycle and playing havoc with the other sensors. My bodge with the gaskets probably made no difference when compared to the open end of a vacuum pipe! One slight problem that I need help with - the valve is held in place by a rather curious fastener. Not sure how it works. There is a centre core which would suggest a push fit application. Certainly not enough room to weald a spanner or ratchet. Any ideas? Would download a photo but can't on this post.