This was my overriding thought when I returned to the house in Crawley after a few days in Edinburgh. The recent warm spell had come to an end and it was pretty chilly outside, but the house seemed far colder than I remembered it. Finally tiring of adding layers I decided (after 36 hours) to look at the thermostat.
Suddenly it all made sense! The batteries in the thermostat had died so there was no heating. No wonder it was so cold.
Batteries replaced, setting reprogrammed and heat flows! Guess I’ll have to check the batteries more regularly in future.