Can I fix my boiler or do I need to call an engineer?

I have a Potterton 80E combi boiler. It sprang into action pumping out lots of heat from the radiators despite the thermostat being set to zero and none of the hot water taps were turned on at the time. Why did it do this? Is it an easy and cheap problem to fix? At the time the boiler misbehaved, I had an underground water leak from the mains supply. Could this have triggered the problem?