It's a pity you haven't said what part of the UK you live in - that makes a difference to when you prune. Very little pruning required, clip off any mildewed growth, remove any dead, damaged stems, trim over to shape. Do it when you see the first signs of growth.

I haven't been able to find much useful info on it. If, as I suspect, it is a once-blooming species, you may be better off pruning right after bloom, especially if forced to give a heavy pruning.