I worked one winter of nights in a sorting office about 12 years ago, it was OK, in many ways like seasider describes, except there was a reasonably good atmosphere, among the casuals at least, no over supervision if your work rate was adequate. It was also just over the road from where I was living and we were knocked off up to an hour early if everything was loaded. Hourly rate was OK, subsidised canteen was good, the only real gripes were it started two weeks later than offered and finished a week or so earlier, at a days notice, so 9 weeks money had to stretch to 12.
Whenever I've needed winter work since, I've found better. Plenty of choice in warehouse/logistics round the E Midlands, though that might not be the case elsewhere. Weekend workers in particularly high demand, nearly all of it agency work, might be worth registering with a few even if they don't seem to have what you're looking for. All crap jobs of course, but I'm quite tolerant of that if it's just for a few weeks and it pays well.