Chickens not performing

My three hens are not laying very well now the winter has set in. It is just below zero here in Suffolk and the hens have grown fresh new feathers after their annual molt, but they are now into their second year and egg production is well down. In the summer we got 18 - 20 eggs a week, but now we are lucky to get 4. Actually only one chicken, Blacky, is still laying so it may be time to replenish the flock.