This is not an answer but another question. When and how did you plant it to make it grow so well, please. As far as your question goes I would say you could "trim" it rather than "prune" it any time. The serious pruning of most plants tends to be done in their dormant period.
hi doris
erm about 6 years ago, in extremely poor sandy soil but in full sun. it likes its feet in shade so I have planted another plant to protect roots from full sun. and away it went...
last year I cut out lots of accumulated brown stems which were basically dead and left the live bits but this was a once every 5 years effort! i would prefer to avoid the brown stems if poss!
I have pruned passionflowers with similar success ! they do seem to thrive in a sunny spot but take a couple of years to get really going then you have to hack them back and then they sulk for a couple of years before starting the whole process again . The answer must be to cut back this years growth each time and try to do it before the frost comes so there won't be any damage .They do throw suckers up quite a lot so pot one of them up if you can just in case of a disaster .
doris - bit tricky to say as I think York quite a bit cooler and wetter than Kent! esp recently! However have had them in the garden when living in the Midlands and they did OK though not as thriving as here. I think the dryness and even the poor soil is what they are used to! Good luck anyway they are very rewarding.
Quattro: thanks too, good point about the suckers, they do keep popping up and my sis has successfully grown one on. So good advice!