Why won't my central heating boiler switch off?

I have a Triancogas boiler which will not switch off with the timer, I have to turn it off at the pump isolating fuse, and as soon as I turn the pump fuse back on it fires up again. I have tried a new timer but this was to no avail, the room thermostat is switching on and off Okay so where do I go from here? HELP!!!!!