I'd do it in late march/april when the weather begins to improve. If it's cut back hard you'll get new growth pretty quickly. This coming winter is forecast as being very cold, so pruning now would cause more damage and risk losing it.
The people opposite are having their big one cut back as I type (well, actually the chap is having a mug of coffee at the moment, but he's done most of it).
We shall see what transpires ....
Pruning to shape is done immediately after flowering as spring flowering plants do it on last year's wood an dthis gives them time to grow new stems and flower buds.
If you are doing a drastic prune and can't wait till they've flowered again then you could do it now, or as soon as the leaves have fallen and the plant is dormant, but you will have no flowers next spring.
Paul, in order to get next years flowers, you must wait until after flowering to prune.