In reply to toasted:
I can also recommend Alec Roberts, he guided me on my first winter outing a few years ago, he's very good.
I would also put in an honourable mention to Sam Leary, although she didn't teach me to climb in winter, she did teach me trad though and I believe she teaches winter courses too. She is an outstanding teacher and I cannot recommend her enough. If you google Sam Leary climbing you'll find her easy enough.