Heating/Water Fault.

[Steve1412]

UPDATE....Finally got chance to spend a bit of time with this. Turned out to be pump over run stat shorted putting perm live to boiler!!! Order new one and stuck it in...luckily this is fitted outside the gas area!!!...Job Done....Thanks for your help all!!!

 

[Barrymk]

i know thus is an old thread but it sounds like the hot water motorised valve is faulty, either the body is jammed and needs freed up or the internal switch is jammed open. removing the orange cable on the valve will stop the boiler firing on its own.

 

[davesparks]

i know thus is an old thread but it sounds like the hot water motorised valve is faulty, either the body is jammed and needs freed up or the internal switch is jammed open. removing the orange cable on the valve will stop the boiler firing on its own.

No it doesn't sound like the motorised valve is faulty, it sounds like the overrun thermostat is faulty.
This is based on the fact that the OP has stated that the overrun stat was faulty and replacing it fixed the problem!