Four-spotted Chaser (Libellula quadrimaculata)
It gets its name from the four black spots, one on each of the four wings. It is easy to mistake this insect for the female Broad-bodied Chaser so 'spotting' those markings on the wing is important.
In many dragonflies the males and females are very different but in the Four-spotted Chaser this is not so, they are very similar and I have no idea which this is although it is probably a male as it had a preferred perch from where it would launch of to deter any intruder that might enter its territory.
Flying from late May until August, you still have plenty of time to find them and, as I said above, the wet areas of the Purbeck heaths are as good a place as anywhere to see them but they can also be found on other still water ponds and lakes that have a degree of acidity and preferably fairly shallow and with extensive vegetation.