British Gas F2 Boiler ignores Timed/Off setting

[tmasa]

Hi, I have a British Gas F2 (Potterton) Boiler which have recently started behaving oddly. It switches itself on at random times, even if the control panel is set to OFF or Timed OFF (both CH and water). There were no recent changes to the system or in the area around boiler.
It does not go beyond the temperature limit set by thermostat if set though.
What could it be ? Thanks

 

[mickyg]

microswitch in a motorised valve sticking, or the valve itself sticking.

 

[tmasa]

I know nothing about the valve mechanics, but if it is sticking wouldn't it just keep the state (like not switching off when OFF button is pressed) ?

It rather switches on and off by itself without any interference from control panel. E.g. timer is set to heat from 18:00 to 22:00, then from 07:00 to 09:00. The boiler goes on at, lets say, 23:15 and works for half an hour, then goes off.

Is it a big job to replace/fix the valve, what is the cost roughly ?