Chaff boiler iving me headaches ?

I have been trying fix a problem with my chaff britony 80 for a while now but today i have given up! i have replaced the 3 way value because there was hot water to the taps but nothing getting through to the rads - this s sorted now - but the ongoing problem is that the heating is now constantly firing even when it is set to hot water only.

today i've changed the regulator pcb having been told it could have been a fault in there but it is still doing the same. I have also replaced the timer/on off switch but this has had no effect. In short, if i want hot water the heating comes on so i have to manually turn off the unit whenever it's not beng used. any ideas?