VCW GB 242 EH Valliant Boiler?

I have elderly boiler above which seems to have a life of its own. For some reason it ignores the thermostat and fires up the boiler despite the wall thermostat saying it is off. I have replaced wall thermostat to no avail and have also replaced the thermostat PCB in the control box again without any improvement.
Tonight with the wall thermostat set for 18.5 C the boiler went to 19C and continued to fire up despite there being no flame displayed in the wall thermostat a new Honeywell Digital

Any ideas what may be at fault
Thanks

David
December 2009