Sorted

The central heating had been doing some odd things, it was supposed to be turned down to only come on at 14C, but had been coming on at odd times and when much warmer. 
The battery in the wireless thermostat / timer was showing as low, even though quite fresh, but touching it caused the controller to crash. Retensioning one of the contacts resolved that and then all I had to do was to reset all of the times and temperatures, and reinstate the offset to make it think that my son's days off (Wednesday and Thursday) are the weekend to give different timings when he is at home all day, not that it matters at the moment as it is back to everything being set to 14C again, but when winter is on the way that will change.