I wouldn't do anything with it at this time of year - you say you've secured it already, so just wait and see what happens. If you find the topgrowth starts to shrivel and die back, then wait till spring and cut it hard then, if you have to. The problem with cutting it back hard is it will encourage lots of growth near the base, and it seems what you want is a specimen shrub on one stem, as it looks now.

What a fabulous amount of berries on it - they'll be contributing to the weight, of course, and I'm surprised the birds haven't taken most of them, if that picture has been taken recently.

Thanks Bamboo for your words of wisdom. You're right, I like it as a shrub on one stem. The picture was taken in August 2011 - haven't had such a show of berries since!