Glow-worm 60 FF will not switch heating off?

I have a glow worm 60 FF with a intermittent fault that results in my heating staying on all night.
I have a external controller programmed to switch the heating on twice a day. Plus a thermostat that also has 2 temperature settings, allowing a lower temperature durring the middle of the day and night. This effectively acts as a second programmable controller.
The boiler always appears to switch of correctly after running for a few hours in the morning but most of the time fails to switch off after runniong for 5 or 6 hours in the evening.
I have tried many different combinations of settings on the 2 controllers to prove they are not causing the problem and I am reasonably sure it's the control card but every time I try to fault find the fault clears.

Before I buy a new control card just wondering if anyone else has had a simillar problem.

Alan
October 2005