Hi, welcome to GoY, theoretically as an evergreen shrub, they should be pruned after flowering, which is in spring, but as this is a hedge, I would say anytime that the shrub is dormant, right through to about the end of july, any later than this, and you run the risk of getting soft growth, which can be damaged by frost, Derek.

I would not prune in the dormant season if you wish to restrict growth. If the hedge is young; prune it in the dormant season to shape it. But if it is growing vigorously, prune it (or clip it) in late Summer (August here). When it has formed a hedge, some people clip twice or three times during the Summer. There are some You tube videos on it but they may be for the States.