God damn it, I don't believe it!
Replaced the thermostat today, and despite reading other peoples posts on damaged O-rings and leaks, I managed to take out a chuck out of an O-ring! Ffs, and I was being careful too! And I only found out after I put it together and ran it up.
It seemed the problem stemmed from putting the 2 short ally pipes back in as they came out (left one came with housing, the right one stayed put). Having both the ally pipes in the engine made it easier to put back, and the o-rings were intact 2nd time.
Oh and I forgot to over-order O-rings, so I had to use an old one :/ (so I'll have to take it off again at some point).
Anyway, warms much quicker now, and stays at ~80C on the highways
, instead of ~60C.