Yellow Rattle (Rhinanthus minor). An annual of grassland, the flowers are yellow and the seeds rattle in their pods. 

 R. minor is a hemi-parasite. It is parasitic on the roots of grass but nonetheless has the capacity to photosynthesise. The vigour of grass is considerably reduced by the presence of Yellow Rattle thereby allowing other species to flourish in meadow habitats.

 For that reason it is often in the mix when trying to establish new meadows with combinations of grasses and wildflowers. The photos above were taken on the Hoo Bit nature reserve last week which adjoins the Pegsdon Hills. I think Hoo Bit was reclaimed from a wooded area rather in the style of wood pasture. Perhaps Yellow Rattle was introduced or perhaps it was present in the surrounding area anyway.