Finally done an oil change. Gone from 10w40 semi synth to API spec 20w50 Mineral in an effort to solve the hot oil pressure issues - the light had been coming on at anything up to 1000rpm with the engine hot, which didn't bode well. The underlying issue is almost certainly that the crank is worn out (it had new shells 8K miles ago, but that didn't fix the problem), but the more miles I get out of it, the longer till I've got to pay for a new one...
The warning light is now out except at hot idle, and then it goes out if I nudge the throttle to add an extra 50rpm or so, which is much better - I'm tempted to knock the idle up to hold the light out all the time.
Incidentally, if anyone is interested I'm still running on SVO, and inadvertently on 100% this weekend... it goes fine at 90%, but at 100% its noticeably down on power... combined with 3.54 Diffs, the peak district, and having a dumper engine belonging to someone else in the back (don't ask), and hills have been "interesting". I've burnt enough SVO off to get £10 of diesel in the tank, which has improved matter dramatically.
As I type
10 years ago