Halstead CH wont turn off

[nholloway]

Rookie here.
Halstead Finest Gold
External Thermostat is set to zero
The timer displays the correct time (ie it revolves)
No matter what the clock CH setting (off/timer/on) the CH is always on.
Powered down the unit when I woke up sweating at 3am and powered back up. All seemed ok. Then when the next CH cycle kicked in at 7am, it stayed on after the cycle 'clicked off' on the clock. I set the stat to zero, I powered off and on again but it remains on CH.
Not sure of it is the stat (don't think so as it shouldn't even get check the stat if if it is set to off on the clock) or the clock or something else.

Any advice very much appreciated.

 

[frankthegasman]

if this is a standard boiler not a combi then the problem is a faulty syncron motor in the zone valve,good luck

 

[nholloway]

Shoudl have mentioned - this is a combi boiler