After doing some research on this specific diesel system and using some very helpful comments on this thread, we carried out some tests to eliminate certain components, What we found was that the pump was unable to build maximum pressure unless the you pick up the engine to about 1500RPM.
with the IMV disconnected on the pump, and the valve on the rail shorted to ground at idle we only seen 800bar before the engine cut out!
Pump was removed by my customer and sent for test (?!) and repair / replacement.
Still have not spoke to the garage to ask in detail what was done but the company sent back a new pump and the vehicle is running perfect!
when i have time to speak to them in detail i will post results..