It flowers on wood made the previous year, so the best time is immediately after it finishes flowering. The shrub has an upright habit so it is doing what comes naturally

I pruned mine in April so that it has plenty of time to produce new flowering stems for next winter.

Such speedy answers! Thankyou Andrewr and Hywel. Your answers agree so I will do as you suggest and do the pruning after flowering which is about April. I love this site!!!!

I do mine in mid-March once it's finished flowering and before the new season's growth begins. It does want to be tall, and I keep mine to about 8', - but I had it much smaller in my previous garden and it still flowered wonderfully. So, you can really do whatever you want with it. It's all in the timing!