My BG 532i combi boilier is not listening to my thermostat?

Ever since I had a 'tracer wire' (?) fitted to my heating system in September (to stop the extractor pipe from freazing over) my boiler has not been working properly. Every couple of weeks or so the boiler stops listening to the thermostat and just goes on permamently.

I have tried turning off the system on the boiler itself and at the mains (this has sometimes resolved the situation for a short time but for no apparent reason why it works sometimes and not others...). I have also tried a comlete re-programme and re-set of my thermostat and I have tried pressing the reset button on the boiler (but this does not even seem to be doing anything).

It's better than having no heating but it's very annoying. Is there anything more I can do without having to pay British Gas to come out?

Thanks