The condensate pipe connects to the underside of the boiler via a 3/4" plastic (usually white) pipe. I would try disconnecting this (it's a compression 'screw-on' type fitting so it's easy to disconnect). Have a pan/bowl handy as you'll get a bit of water coming out.
When the boiler runs you'll get a drip/trickle from the connection, so keep a pan/bucket underneath until you know the external pipe has de-frosted and you can re-connect the condensate pipe to the boiler.
Good luck ok!
CMac
December 2010