The message about the injector was passed on to me today, I haven't been back to the airfield to talk to the engineer in person. Before I do go back, I'll certainly be reading up on how injection systems work - my old text books just show carburettors.
The engine continued to run smoothly, with the reduced power. and in response to an earlier post, only AVGAS from a certified source is used for that aircraft - no way could it have been MOGAS and no one uses the aircraft for touring so it never leaves its home base.
I'm just glad 2100rpm was enough to keep me flying and enough for a slow climb to give me 2500' qfe when I got to the field. Phew!