Update to this problem.
The dealer didn't want to know. So, after a lot of going back and forth to my local indie garage i was no further forward and the prolbem was getting worse. I took the car out for a long drive to see what would happen. The engine light came on!! (never thought i'd be happy to see one of those)
Back to the garage, code has shown it's the Fuel Rail Pressure Sensor that's faulty. Car booked in today to get it changed.