woo! yay! Just managed to get through to British Gas, and they are sending an engineer out to me tomorrow morning. Let’s just hope he can fix my boiler straight away. Having gone through the diagnostic procedure in the manual, there are two likely causes for the problem – either some debris in the system that is blocking it, or (more likely) a defective thermostat. If it is the thermostat, I guess there could be a delay if parts are required.