Thanks so much for coming back to me. It’s downstairs in the hall. My dad has turned off the radiator in that area and there are no hot water pipes near by (or in the wall). It’s also one of the coldest areas in the house (whilst he’s testing it). So (in our mind), it should read a little...
My parents have recently had British Gas install a new boiler along with a WL2 thermostat. The problem seems to be that the stat goes up very quickly (and then switches off) but doesn't come down correctly. This stops the heating from coming back on when the temperature drops below the set...