if the microswitch is sticking then you really need to replace the motorised valve actuator. you may have 1 or 2 of them depending on the system design, but i'd suspect you have 2 going by the way you desribe the fault. the motorised valves would usually be near the pump, they can really be anywhere but most likely in beside your hot water cylinder.
what the valves do is let you choose between heating and hot water by directing the hot water from the boiler around different parts of the sysyem. when the timer (and thermostats if you have them) are calling for heat, they send power to the valves, which motor open and knock on a switch once opened fully, the activation of this switch sends power to the boiler and the pump to fire them up. so even with the timer off if this switch is on, the boiler and pump will be on. the only way to turn it off until it's fixed is to turn the power off to the heating system.
hope that helps
Ian, January 2010
