I would assume a jammed valve too.
Stuck in the position to send water around the radiators a request from the programmer for hot water fires up the boiler and sends the heated water to the wrong place - the radiators - and not to the hot water tank.
Normal control to turn the boiler off should come from the thermostat on the outside of the hot water tank but the hot water tank is not getting warmed up - the tank stat remains closed, the boiler continues to burn and heated water continues to be sent to the rads which get very hot.
Very unlikely to be the timer/programmer or a wiring fault.
Peccavi
March 2010