Yeah, they lay eggs in winter. They just slow down because the lack of light. The only time that they don't lay eggs at all is if they're molting, and that's when they lose all of their feathers and grow them back again. That takes about 3 weeks, and they'll do that next year.
A heater would definitely help. We should have a heater in there in a few weeks here.
Didn't know chickens laid eggs in winter. Would a heater help on cold days?