I cannot remember noticing these exquisite pink flowers before. They are Corncockles (Agrostemma githago). There are only a few of them growing in ones and twos deep amongst the flowering grasses and the stems of cow parsley bearing green seed heads. They are difficult to spot, especially when the wind is blowing and the vegetation is swaying. The wind also makes it difficult to get a good shot. You have to wait patiently for the micro-second of calm between gusts before pressing the take button.
Corncockles are described as formerly widespread and common but now extremely scare and erratic because of agricultural herbicides. In this location, the nature reserve at Charlton Down, they have probably been seeded deliberately at some time.