Central Heating comes on with hot water?

I have a Landis and Gyr electronic timer (seems fine) attached to a boiler which provides hot water via a hot water tank.
You can have the hot water turned on by itself, or have hot water and Central heating together.
Today putting the Hot water on (by itself) the radiators have all become hot (and the central heating system is off) the thermostat is turned down but this has not helped.
Any suggestions?