Ok, just to round this one off ... new thermostat seems to have solved this. On a short run after replacing, it sits at 90 rock solid, no movement.
Plugged into AlfaOBD the temp fluctuates between around 86 and just over 90 while on a run.
Thermostat change was easy enough. Battery had to come out, and I attempted to remove the tray too, which would have made it easier. However, the rearmost bolt wasn't for moving. Managed anyway.
The two 13mm stat bolts can be reached with a short socket extension, easier with a 3/8 than a 1/2. You're not working blind for the bottom bolt, because you can see it from the front once the big hose is off.
Hard plastic pipe has to come off the inlet manifold beside the EGR valve, three hex boltt, two of which also hold brackets on. Can then be shoved out of the way without removing completely.
Old hose clips come off fairly easily with a screwdriver and controlled violence. The one behind the temp sensor is easier when the stat is free of the block.
Coolant system appears to be self-bleeding. Fill up to Max, run the engine up to temp. I went out for a run, with the reservoir cap on, otherwise it'd never have got up to temp in this weather.
Not a difficult job at all, as it turne out. Took me about 2 and a bit hours, taking my time, drinking coffe, and cleaning stuff as I went along.