Great success!
Conclusive proof its the MAF giving bad readings, despite no codes.
Earlier today I unplugged the MAF to see if it made any difference. It didn't. I forgot to erase the fault code - that's important...
Reading up a bit more about the O2 sensors in ElsaWin I found the procedure to re-learn the adaptation values (basic settings 32 & 33), and also how to turn them on and off (basic settings 99). So, over to the workshop and do those, and immediately it starts running nicely
All O2 values in range (despite being -25% earlier today), and a pound coin balanced on the inlet manifold for ten minutes without falling over. Rpm rock solid at 760.
Bonus. Oh, and clear the MAF error code while I'm here. Immediately rpm drops to 650, cough, cough, cough, runs like crap! Check the O2 values and they're pegged at 25% again. Turn the O2s off and it barely runs at all. Un-plug the MAF again, re-learn the O2s and it runs fine again. As soon as I clear the error code, back to crap.
Interestingly, engine load is just in the zone with the MAF unplugged too - 20%, with a range of 14-20. That seems plausible for a tight new engine and the rad fan running.
A call to TPS in the morning methinks
Oh, and Goran I owe you a
for pointing me in the right direction!