So, after dumping all its coolant in one big heap at motorway speeds, a colleague (who in a former life was a race engine tuner) and I had a quick look, and quickly determined it was the stat housing had failed. Nothing repairable at side of road, as its a unique, plastic, moulded part. Good job I have RAC recovery.
So call RAC, to be greeted with a message that I can't really call them, and report it online instead if possible, because of covid, and only hang on as a last resort. I mean, we've had the sniffle virus for over a year, companies can't keep hiding behind that excuse.
Gave up holding after about 10mins, and did it online, at 2:30pm. Technician called me at 4pm, telling me he was 30mins away, though I explained to him what I'd found, and that it was gonna need recovery, and it was RWD, but understandably he had to check first. He rocked up at 4:30pm, and immediately decided it needed recovery, which he couldn't do, because it was RWD. So I had to wait for a recovery truck.
Recovery truck turned upat 7pm, and was quickly on our way. I had to stay in the car, due to covid, an experience that was quite unpleasant.
By 8pm, he'd dropped me at Beaconsfield services.
By 11pm, the next truck turned up.
Which meant I was home just before midnight.
This afternoon, after work, I pulled off the throttle body, to reveal fully the extent of the problem. Its actually not the thermostat housing, just a plastic component to send the coolant in various directions. The stat is not actually on the engine, and is mounted remotely.
Now time to order bits. Need to decide genuine or pattern. Genuine is about 2.5 - 3 times more expensive, but its not an expensive part, so will probably be based on availability.