Why do I keep getting water in my condenser pipe on my Worcester-Bosch 28i ?

Hello and Merry Christmas everyone,
I have got a Worcester-Bosch Junior 28i combination boiler and periodically it purges and causes water to flow from the condense pipe. This normally isn't a problem apart from when it freezes like it did Christams Eve, creating a lovely present for me to rectify on Christmas morning by means of removing the external condense pipe (that was full of ice) defrosting it out and subsequently defrosting the internal pipe it was attached to by use of a hair dryer, then replacing the external condense pipe, topping up the water system and restarting it.
So, does anyone know why this is happening to begin with and is there anything I can do to fix it as opposed to calling a heating engineer out?
Thanks, Ivan.