I think heaths (Erica carnea) as opposed to heathers (Genus Calluna) are hardier. Many of the cultivars are okay for zone 5: 'King George', 'Vivelli', 'Anne Sparks', etc. I'm not sure of the scientific differences between Erica versus Calluna, but the plants look similar.
Many thanks, I shall look for those. I have been wanting to plant heather ever since I made my first visit to Victoria thirty years ago and saw so much of it everywhere.