I didn't read the post properly, as said I doubt you can adjust the idle speed that high. iirc the increments are 25rpm but not sure on the maximum adjystment.

Is it possible (I know some vauxhall diesel's do it) that the ecu has gone into emergency running mode so holding the engine speed higher than normal.

I had an astra y17dtl which idled at 1500rpm due to the ecu being faulty which lost its throttle control so switched to emergency running mode. Also had fault codes listed but you haven't mentioned any.