Once you know the three common heathers, bell heather, ling and cross leaved heath, you will then be able to recognise the fourth main heather species which is much the same but quite different! Dorset heath (Erica ciliaris) is a nationally rare plant but it can be locally frequent in the Purbeck area of Dorset. It does also occur in South Devon and Cornwall but Dorset is its stronghold. It is quite common in parts of southern Europe.
This is a heather that likes it damp but not wet. It tends to grow taller than the other heathers and has tapered rather than the bell-shaped flowers of bell heather. It is a much deeper purple than ling and has far more flowers than cross leaved heath. It flowers from July to September. There are sub-species that result from hybridisation with other heathers but identifying those is best left to the experts, it is way beyond my abilities!
In a poll organised by Plantlife back in 2002 this was unsurprisingly voted the county flower of Dorset.