Just to add weight to the barage of "worked for me too" responses...
The kettle trick worked for me too.
Since it seems there are hundreds of people with this exact problem, in cold houses on snow bound roads, that plumbers might take days to reach, I'll add a few bits of info that might help.
The condensate discharge pipe is usually plastic, white, grey or black and quite narrow, running from the boiler, outside the property and into a hole in the ground.
Remove any insulation, pour on boiling water, replace insulation.
Pour the water very slowly, you are looking for contact time not volume. A whole kettle full dumped on the pipe in 5 seconds won't be very effective because very little of the heat will be transfered to the ice.
Boiling water poured slowly onto a thin pipe should clear the ice pretty fast, well within 5 minutes. I heard cracking ice in seconds and gurgling water in 2 minutes.
If it is taking longer than this it is more likely that you need to work on another section of the pipe, than that the section you are pouring onto has magic ice that can withstand 5 minutes of boiling water.
Start at the drain and work back towards the property, the ice will probably have started forming at the furthest point from the property.
If you need any more help stick the the following into a browser:
worcester bosch greenstar fault code ea
Good luck
Alex
January 2010