Chalk Milkwort (Polygala calcarea)
Whilst Milkwort is quite easy to identify, separating the three British species is not so easy! The differences are quite small and it takes a good botanist to spot the minor variations between them.
For the lesser botanist like me then habitat is the best guide but certainly not the most accurate. In general however, Chalk Milkwort grows amongst short turf on chalk in southern England; Heath Milkwort ... yes, grows on acid soils and especially heathland and is common on the Dorset heaths and elsewhere it is likely to be Common Milkwort! Common Milkwort does also occur on the chalk as well as heathland though!
Just to add to the "interest" Heath Milkwort is also known as Thyme-leaved Milkwort. Not only that, all the species occur frequently in a mauve colour as well as the lovely true blue, quite often growing quite close to each other.
Regardless of which of the species it is, in my opinion it is well worth getting down on your knees to have a closer look at.