Am not much of a fig tree expert Alec although I do have a small one in a patio container. Mine, however, has all the growth around the bottom and has one silly main shoot about 4 feet tall with a few leaves on it. Figs apparently fruit better when their roots are restricted so keep it in the pot but move it out of the greenhouse. Mine has survived two winters quite well without any protection. If you need to prune the top out to make the lower branch stems develop I suggest doing this in the autumn when all the leaves have dropped for the winter. But make sure there are some little nodules lower down the stem where new shoots can start growing in the spring. I think once you prune the top of the plant off any figs on the pruned branches will die but the remainder may survive.