In reply to Ban1:
Personally, I only go bouldering in winter. I find it really hard to have any fun at all doing trad in winter, and equally hard to have fun sport climbing regardless of the season.
I think spending a chunk of time bouldering is good for your climbing, and for me it can be as rewarding as trad (ok, only occasionally) - particularly days climbing a high volume of highball/micro-route type things (I find working the same 2 moves all day about as fun as going sport climbing).
For me in winter the days are too short and it's too cold for trad, and since gritstone lends itself so well to bouldering - and the hard stuff is impossible in summer anyway - it seems like the logical decision.
Can you trad climb in the middle of winter? Yes. Is it fun? Matter of opinion.