Cowslip

Primula veris or Cowslip is coming towards the end of its main flowering period now. It is closely related to Primrose (Primula vulgaris) and hybridises with it to form False Oxslip or Primula x polyantha. 
However, unlike Primrose, which favours shadier spots and flowers a little earlier, Cowslip is a plant of grassland and the open. It can appear in vast numbers, competing with Dandelions for the cones in your retina.
There was a nice article by George Peterken in the current British Wildlife magazine about Primroses which revealed a great deal about a species we very much take for granted.