Hi Bob, this charming shrub is a very good seller for me here in west cornwall and this is what I advise my customers to do. For the first couple of years, prune it back by half in the spring and then the subsequent seasons growth again by half in the first week of August. By doing this, you form a dense bush which will be better able to stand cold weather. The problem is that they flower just about all year and so people are loathed to prune them as they always have flowers somewhere ! I have quite a few customers from your area (mostly holiday makers) and I would advise growing it in a container for this initial 'bushing up' stage and planting it out when it was a little larger and bushier, of course, ensuring you have the required acid soil, in which they do much better.
regards, Lannerman.