Just in case anybody reading this is curious. To get the bottom bolt of the thermostat pipe off (without moving the alternator) required the aquisition of the smallest 10mm hex head socket on a universal ball joint I could find, £6.30 well spent in Murray's tool store Edinburgh. Thermostat now works as expected and temperature gauge sits at a solid 90.
Except... I overfilled the coolant header tank, and on the first proper journey burst the temperature sender out of its seal :embarassment: Either too much pressure in the coolant system, or more likely I'd insufficiently seated the temperature sender when I replaced that previously.
All now the way it should be and hopefully stays that way!