Boiler wont switch off?

I have a British Gas/Glow worm 330HE boiler fitted in 2006. Over the last few months a fault has been occuring. The system works fine normally I can turn the boiler one/off with the thermostat when the programmer is on but if the boiler happens to be running when the programmer goes off then the boiler/pump will not switch off and carries on heating the whole house all night! I can turn the boiler off and when I turn it back on it starts up again, I have to leave it off for several hours before it works properly again. If i turn the programmer back on then turn the stat down the boiler cuts out as normal, then when the programmer goes off all is fine. I am thinking boiler control unit? Any advice please?