Kev, we changed all filters first time it came in so they are OK (the air cleaner was well clogged and was not in the housing correctly so not sealed anyway)
I have driven it about 40 miles and no problems at all
I am inclined to think you are right about turbo, difficult to prove with it being so intermittent. I will have to speak to the customer and give them the option of running until it is a more consistent fault, or fitting a turbo but can't 100 % guarantee that it will solve the problem
I had the grill & head lamp out to check wiring for chafing etc all OK & checked boost pressure valve is reading correct, so I think I have taken it about as far as I can at the moment.