Summer flowering heathers should be pruned in mid March, cut the stems back to just below the faded flowers, removing most, but not all, of the previous season’s growth. It’s very important to avoid cutting back into old wood because heathers do not readily produce new shoots.

Winter flowering heathers are pruned after flowering is over, so April time generally, again do not cut into the old wood.

Yes Mg is correct. In the wild they are very bushy as they are grazed and the sheep etc tend not to eat into the wood too much.