I think you should do it in the Autumn or Spring, after the frosts have finished, or before they start (Autumn). They don't like being moved though (who does?) so you need to dig up a big root ball with plenty of soil around it, and have the hole already dug, where you want to replant. Then give it loads of water, and some ericacious fertiliser to keep the soil acidic. Hope this helps. Other members may have other ideas. Good luck with it. Annie (Cumbria)
P.S. Don't plant it where it gets the early morning sun, by the way, or strong winds, as it doesn't like either of those, apparently.