All heathers are shallow rooted, Jonathan, and should be planted at the same depth as they were in the pot. They will make a broad mat of fine roots just beneath the surface. Yours look just fine in the picture.

There is no need to plant them deeper but I used to grow 10000 Erica Darleyensis per year and I find you'll get a bushier plant if you do.

A couple of those in the photo look a bit open and leggy. I would dig them up and replant them about 2 inches deeper and push the foliage tighter into a ball by pushing soil against it. The foliage touching the soil will then root into it.

As an aside. Erica darleyensis take fairly easily from 1 inch cuttings but if you spread an inch or two of soil/potting composts over the top of the plant,shake it down to the roots and keep it damp they'll root into this in a month or two during the growing season.

You can then divide quite large rooted pieces up from the parent plant as you would dividing a perennial. ( dig up the whole plant and pull or cut the rooted pieces off the plant.

Never let a newly planted heather get dry. If it gets dry once it will die.

It does seem logical to plant them deep if they can do so, especially if they are shallow rooted. Makes sense. Will do. Many thanks. I dug plenty of soil conditioner in.