After ringing two plumbers and getting no answer (for some reason they all seem to be busy ) I resorted to youtube and, lo and behold, got it sorted in ten minutes. Turns out that what they call the "condensate pipe" was frozen. This is the pipe that takes excess water away from the boiler and often goes outside the building. I traced it outside and surely enough it was surrounded by ice. Boiled the kettle once, poured it on the pipe from top to bottom and a four foot block off ice slid out of the bottom (after I'd broken it off in three inch lengths). Well chuffed not to have to pay a call out charge and waiting all day for a plumber to get to me in the snow. Log it in your memory banks chaps. It may help you or someone you know next winter.