Heating not switching off when set temperature reached

My heating is controlled by a timer and also set not to exceed 20 degrees. After running fine for a few days, turning on and off as it is set to do, it then doesn't turn off as it should, so we come home to a house at 25 degrees. When it does this, the only way to cool the house is to turn the boiler itself off. Turning down the thermostat or turning it to off from auto doesn't stop it. The boiler just keeps on going. It is a 13 year old Potterton boiler and a Megaflo tank. The boiler has had a new mother board in its lifetime and the whole system was serviced in March 2013, about three weeks before the fault appeared. Three heating engineers have failed to resolve the problem. The third suggested a new froststat, which didn't help. The boiler, froststat and pipestat are all in the integral garage.