Yay, I think (I hope) I've fixed it. Just hope I'm not speaking too soon, "fixing it" has happened before...
Anyways, I thought I would delve into the EHT board and test the power transistors, and go from there. Pretty quickly found a 2SB1417 which appeared to have an open emitter. Pulled it, tested again to make sure... Nope, out of circuit it tested fine. Soldered it back in, tested fine again. I checked all the other semiconductors on the board whilst I was at it - all fine. Put the board in the printer, cleaned out all the dropped toner... test print... Yay! Shiny Red Car means it works!
So, in what I hope is the conclusion to a fault that has been troubling me for over a year now, a dry joint in this circuit would have produced the erratic and intermittent and apparently temperature sensitive fault I've been seeing.
If you hear a scream, you'll know it's doing it again though...