Thanks for the reply. I'm afraid none of the above apply! The pipe is completely clear, leading to a drain the sits just outside the wall where the boiler is located.
Could it be possible the tank itself is faulty, and not draining as it should?
I've got a problem with my combi boiler (hydroline B24 - a truly miserable piece of crap). The little bung in the condensation trap with electrodes in keeps tripping the boiler as the trap is getting too full. Last time this happened my pipes had frozen and the heating engineer that came out...
