Duff thermostat
I bought a Gates unit since we're trying to limit expenditure on this one, but its either a Chinesium part in a Gates box, or just a straight-up counterfeit because on inspection there are no Gates markings or part numbers on it at all. When I fitted it, I just took it out of the box and put it in the engine, because OE manufacturer. It works in a pan, but not in the car.
Lesson learned. Saved £15 to waste 8 hours