UPDATE!

Awesome news, and a good example of never automatically thinking the worst!

Today, just to see if it was possible to stop it overheating enough to drive it to the workshop, I removed the thermostat...

It was FUCKED, the inner section looked like a bullet fired from a gun, with bad score marks all along it where it had been scraping on the outer section trying to move.
It was a proper Motorcraft one btw.

Removed it, bled and refilled the system, all seemed fine.

Drove it for ~20 miles inc a few squirts (inc up to 5k in 5th) and it was fine, didn't get hot, didn't do a thing.

Once got back, turned it off and it didn't try to bubble over, the header tank level was as high as you'd expect it, all seemed ok.

After a minute we undid the header tank cap (bearing in mind this was a 2bar Audi 1.8T one so could go off like a bomb lol) and the pressure was very low, in fact less than in any time since I've owned the thing.

So yeah, it seems, it was purely a massively shagged thermostat that was getting worse,
and the heat alone was causing the pressure and bubbling and so on.

Good old YBs too, reminded me of another reason I love 'em- Most engines usually crap their pants at the first sign of overheating, but nope, it's loving it.

Hopefully at least, that is that...