I had the same problem, twice now and is a common occurance.The condensing pipe from the side of the body of the boiler, which I assume is fitted to emit to an outside drain, is totally frozen, have called out british gas with home care and they are forever getting call outs to this problem, they even lagged the pipe the last time it froze and it still happened, The engineer informed me they are teasting a product to eliminate the problem.make sure when you have de iced that the pipe points downwards.
Ken Rowley
December 2010