Do you mean Leycesteria formosa? If so, it's not tender in N. Yorkshire, and is quite happy to seed itself about.
Pruning - in theory, you can cut it to about two foot, but I think it would be better to leave it alone if small - pruning can cause stems to die back, and might harm it if not well established. I tend to cut out dead stems every year, and trim excessively long ones by about half - it took a long time to recover from the only hard prune I gave it.