Having raised this issue months ago and still no cure I've now learnt a bit more from a new diagnostic scan so hopefully someone will have the answer.
At idle of ~770-800 rpm the #1 injector correction reads ~140 with the other 3 all ~70 and it is running a bit rough with a flat spot when accelerator is slightly depressed. As the rpm gets up around 1400 the #1 figure decreases as the others all rise till at 1500 every injector has a correction of 98.4 and the engine is smooth in the workshop and good to drive on the road.
Almost 2 years ago the engine began to randomly have a momentary (milli-seconds ) power loss. It was totally random and not able to be found by dealer scan tool. On the day the rough running began the power loss moments became very frequent until at about 85-90 on the highway it suddenly lost power and began to run roughly while idling on the side of the road. By using the oil change reset procedure it resorted to normal idling and performance for another hour of highway travel, but next morning it was rough as at low rpm. Dealer scan shows #1 injector to be overfuelling, but testing says it is OK and swapping them does not alter the readout as being #1. So logically it seems the ECU is instructing the injector but only till ~1500 rpm.

Any thoughts will be most welcome. Thanks