when I had Powis Castle, I trimmed in the spring. My belief, for many plants, is to let the plant survive well with no fall trimming, then trim in spring as needed and to remove any winter dieback.
If you trimmed too much in fall and you got a severe winter, you might have to trim still more in spring. Powis Castle is a very hardy plant to start with, but you just can't know how the weather will be these days.
For my plant, it eventually outgrew its area and I ended up digging it out and tossing it as it became quite enormous and I wasn't in the mood to fuss with cuttings for smaller plants - for a short term.
