Can anybody suggest a means of lagging the pipe and with what material?

Re Worcester Bosch Greenstar CDI Boiler..
I have yet again had a problem with the condensate external pipe freezing up and consequently the boiler not working properly so no heating of radiators or hot water. We solved the problem this year by pouring hot water over the plastic downpipe thereby thawing the pipe out. We think that the pipe is not thick enough to stop the frost getting into the condensate inside the pipe.

Question: Has anybody got any ideas on how we can lag the pipe and with what material to prevent it happenning again?