Strange temperature behaviour...

stat is definitely the first thing to try.

Replace the stat and whilst at it, make sure that there's no air locks. I had a similar issue with a Mk1 and the water would migrate around the system - sometimes giving warm air at the vents and at other times actually trying to cool down the engine like it should.

OK, Fair enough. I've got a stat with an air vent (goes at the top, I know...) so I'll stick that in at the weekend and see how it goes.

I haven't heard of a particular problem with airlocks on these, as opposed to, say, almost all Peugeots... do you know of one, or is that general advice?

I can only speak from my own experience with the Mk1 Mondeo - and, I did have an air lock (how it got there, I have no idea).

If a Mk1, then also check the steel water pipe under one side of the engine. If this rusts through (as did mine during an MOT) then it's worth replacing at the same time. I swapped mine with a short length of microbore, I think - it was many years ago.

Useful info, thanks. I'll check it out.

Replying to my own post. Johnny No-mates, that's me

For the benefit of futureGooglers...

I replaced the stat and it all seems absolutely fine now.

My theory is that it was working up to jamming open, but hadn't got there yet. So it was closed at startup, opened normally, but was very slow to close again. So for the whole trip it was holding the maximum open it had seen for a while (hence overcooling), but after I stopped it could shut slooowly so it was closed again the next morning.

That's believable enough for me, anyway. And it works.

I will file those symptoms for future reference.

Mrcheerful