How do I fix my bosch worchester combi boiler 24CDi?

I have just come back from holiday to a really cold house. I put the heating on for the first time in 5 months and the thermostat isn't conecting to the boiler. The signal light is on constantly ( normally it flashes every 10 seconds). The heating won't go on unless I press the over ride button but I know I can't keep doing that. I'd rather not call british gas out unless I have to as its a £50 call out fee. I have changed the batteries in the thermostat but its made no difference