Hi Colin,
I'm in South Texas, where the climate is a little warmer than England. Best time for flowering is during the summer months. Then 2-3 months after that, provided things stay warm, is ripening. Here's some tips - after the flower stem has fully emerged, cut the male flowers. Thus, all the energy goes into the female banana hands. At the first hint of yellowing in the upper hands, cut the stalk, and hang in a cool place - 2-3 days, and all should ripen. Never let the hands completely yellow on the plant - they will split.
What type of banana do you have? They like heat so you must have had a mild winter. I have Raja Puri growing in my yard - have about three stalks ripening at present.
Gene, in South Texas