I somehow managed to damage the temperature sensor in my thermostat while cleaning corroded battery contacts. Apparently the only fix is "buy a new thermostat", though it is a pity to replace this for a component worth a few pennies.
This is a "British Gas" branded RF-linked Drayton Digitstat thermostat.
The temperature sensor seems to be a two-pin ceramic device with blue body and white tip. As far as I can tell there are no other markings, and there is no standard colour scheme.
- before desoldering I checked the connectivity, and it seems to be connected to the traces
- after desoldering I measured resistance, and room temperature I've got ~55kOhm. Holding it between fingers it goes to ~45kOhm