As above, working in winter, so far, has not proved much of an issue. I may well get held up for a week in extreme circumstances, but i just get as much as i can done, when i can!
Oh, and i don't have a hot water system 
Not been on the go long then Simon?
2009-2010 was pretty bad really bad effing terrible. The mercury scarcely went above zero for months. Very nearly finished us off.
Make hay while the sun shines, rain pours, wind howls, 'cause when it gets to -22 (not that that is very likely in Cornwall) you aint cleaning much.
Yes, i remember the winter you mention, but a couple of things made it not such a disaster. Firstly, the bad spell came over christmas and new year, so although i (as i'm sure was the case with you) couldn't get out for about 10 or 12 days, i would only have been trying to go out on about 6 or 7 of them. Also, as that was my 1st winter as a window cleaner, i did not have a huge amount of work at that time anyway, so it did not take that long to catch up. But i remember speaking to another local window cleaner i knew who had a much bigger round, he said he had to write off a large part of that months work.
I seem to remember the following winter had a couple of pretty bad spells, where i couldnt get out for about a week each time
But the way i look at is, if i am late, i am late, not much can be done about it. I cant see me writing off a large chunk of my months work, i will just do it late. I have done this a few times now, if, for whatever reason i am, say, a week late, i will just do the work, but the next month, if i can, i will do a couple of days earlier, and so on, till the work is back where it was. This is one of the reasons i work on 'an approximately per calender month' basis, which is what i tell my customers. It gives me a bit of flexibility for winter delays, holidays, days off for unexpected reasons, or whatever. If i worked (as a lot do) per 4 weeks, i would be under much more pressure to get the work done on a particular day.