Pete, I have read all of the post this with interest. Sounds like your having a nightmare.

What I would be doing is looking EOBD, see what codes you have there and look at the freeze frame data. (Can you post this info) This gives you a bit of info on what was happening at time of fault. The light is probably coming on after a week or two because of the drive cycles. Have asked him what conditions the fault appears. What Scan tool are you using? And have you done a smoke test on inlet manifold

Regards Darren