My gas boiler is in a 'side loft' under the eaves of the house, a cold but not freezing location even in current weather. It is controlled by a Honeywell ST7000 programmer (basic but adequate) and there is a Honeywell frost stat fitted just above the boiler. All has worked well enough for some years. The programmer is set to switch 'off' at 10.30pm but on several nights recently the boiler has continued to run. The programmer is not then showing 'on' and, to turn the boiler off, I have to use the override button twice to show 'on' and then 'off'. The overrun has not occurred at the morning switch off time. I am very doubtful that the cold weather is triggering the frost stat - and why should it when the boiler/system has been running for several hours? But, if the frost stat is operating, surely I would then be unable to switch off using the override? Perhaps the programmer is faulty (sticking relay?). Any pointers would be appreciated so I can investigate further. There is no pipe stat on the return (in series with the frost stat) and maybe I should fit one - but that has no bearing on my present fault, does it?