Potterton Suprima - continually runs!!!?

Hi, My boiler ha started to run continually. My only way of controlling it is to switch on or off by the wall supply spur.
I have removed programmer but still it fires up! On reading someones similar query the suggested the motorised valve may be the problem, I disconected the valve motor Ican move the actuator by hand using the manual slider so prsume valve is not clogged up, system is only 8 years old, and with the programmer still removed switched supply on and it still fires up!!!!
The only thing in the aring cupboard I haven't looked at (because don't know what it is or does) is a piece of kit with the words myson power Extra n the white plastic cover!
Anybody out ther please help!!!!!!!!