If you're worried about windrock, then take a little off, though I'd be inclined to wait a bit longer so you don't force new growth just as winter starts. The time to prune these is actually around March/early April, as growth begins, when they should be cut back quite hard.

Thanks, thought as much! Only one plant in an exposed area, so will hang on till I'm sure it's asleep. Neighbour has suggested I put some small stakes in to give it some extra support.