The only info I can find on pruning these suggests doing it regularly - it flowers on new wood, so it shouldn't affect the flowering. In general, it isn't a good idea to prune hard any plant which is going to be outside all year at this time of year, unless its already dormant. If you've not got this as a conservatory or houseplant currently, you will need to move it to a frost free area for winter, it's not fully hardy in Zone 8, which is most of the UK.