FIXED!
Sorry abut delay in posting fix, my customer has been to Cornwall trying out the car, he has reported back all is well now.
We rigged in a vacuum guage into the actuator hose for turbo, and ran it in tandem with solus showing desired boost and actual.
Eventualy we could predict when the fault was going to occur,generally when boost reached about 500 mb over desired, this would coincide with a momentary blip in the vacuum line to the turbo.
By a process of elimination disconnecting the other solinoids from the vacuum system we found the solinoid for the intake tract was leaking vacuum, swaped it out temporarily for a working N75 valve and the fault was gone!
Thank you very much for all help.
Guess what?
Got another one in next week, this one has aso had turbo off and cleaned!