Weeel ladies and gents... the aircraft is back on line and sure enough it was a right mag problem.
The oil seal between the back of the engine and the right mag had cracked and allowed oil into the mag which had pooled and "drowned" the sensor giving eroneous readings to the governor (which then tried to open the throttle wide when we were already at 102% engine rpm).
All fixed and thanks very much to all who helped point us in the right direction