I had my block paving done Jan 2010 when there was still snow on the ground and freezing most nights. Didn't to seem to make a blind bit of difference except to the 3 poor guys doing the work being consatntly cold. At one point though, all the melting snow drained down to where they were trying to put a soak-away and it just kept filling up, so one weekend I spent ages draining all the water out for them (with a bucket). That had nothing to do with the paving though which still looks as good as it did now as when they first did it. I had a wall built too, but the frost one night meant the cement didn't set properly, but they simply did it again. I'd say go for it as it will be a lot cheaper than spring/summer and it keeps guys in work over the quiet period.