Depends which heathers - many of the summer and autumn flowering ones require acid soil conditions, particularly Calluna varieties, but three of the Ericas (Erica carnea, E. darleyensis and E. erigena) grow in most soils.

Its probably easiest to just buy ericaceous compost, then you don't have to work out which ones you can use where....

Thank you, Bamboo. Yes, it's probably best to buy the proper compost. I'll do that. ?