I had exactly the same problem a couple of weeks ago , My system was put in about a year ago and we did not have any issues last year but the freezing temperatures this year stopped the boiler dead as it reached -17 it simply could not get rid of the condensor chamber fluid, I decided to upgrade the condensor pipe to 32mm , give it a steeper decline going into the drain and also I put in a extension pipe with an end cap which will allow me to use it as a funnel should i ever get it freeze up again , I will simply take off the end cap and use the extra section as a funnel to pour hot water down , Fingers crossed however it has worked fine since I upgraded the pipe , With hindsight the original plumber should have run the longer part internally before exiting the building into the drain but I think he took the lazy quicker way to be honest .