My hot water boiler failed **On** - what would cause that?
Despite scouring the Internet, I didn't find the solution - the gas man came today and after describing the symptoms to him he identified it immediately:


Faulty zone valve contact switch


Basically, the sequence is as follows:


Timer turns on - sends current to thermostat
Temperature is too low - thermostat relay sends current to zone valve
Zone valve turns fully on - contact switch triggers to instruct boiler to fire up
Timer turns off or temperature reaches threshold - current no longer passed to zone valve
Zone valve turns off - But faulty return spring kept switch on instructing boiler to remain on


It turns out that these zone valves have a plastic casing, rather than a metal one, so the return spring hooks around a small plastic spur... which just snapped off. 

The fix: popping in a small screw to hook the spring round

Callout charge: £90