It was a tad chilly when I got home last night. Although there doesn't appear to have been frost here, it's remained quite cold in the house. The central heating doesn't seem to understand the 'on' light. Or rather, it only seems to understand it when the hot water is 'on', too, and as the hot water tank doesn't get drained all that much, the timer for it is set more restrictively. It didn't previously behave like this. The boiler is clearly fine, and the valve and pump for the heating too, so maybe there is a problem just with the controller. I'll try resetting it later when things have warmed up. If resetting doesn't sort it out, I'll call the plumbers on Monday, but at least there seems to be a way of ensuring the heating is on.