A simple 2:1 mixture of granulated sugar and water is sufficient. I've used this mixture for years with up to 10 hives here in the western U.S. Avoid fedding honey, since it may contain bacteria causing Nosema. We use a simple bee feeder obtained form an on-line apiary supply. It holds a normal sized quart or pint fruit jar (they standard type used for canning) and will last the two frame hive about a week and a half in the winter. It's important to feed sufficiently to keep the bees from becoming to invigorated to where they start flying from the hive.
I've also used a product described here: https://www.dadant.com/catalog/product_info.php?products_id=824 when it's to cold for the liquid since it can be fed in powder form. I realize this is U.S. based web site, but a bee-keeper supply should be available in the U.K.
Most importantly, do not harvest to much honey in the fall. In the wild the hive lives off the stored honey from the summer season, so, as much as possible sufficient honey should be left for the bees to over-winter successfully. Even with the feeding, we expect to lose about 20% of the bees due to our harsh climate (It's -8F here this morning)... Best of luck