Sorrel

Amongst the buttercups the rusty red flowers of Common Sorrel (Rumex acetosa) 

stand tall and tint the meadow.  

You can tell it's a Sorrel and not a Dock because the leaves have little curly horns where the leaf joins the stalk, sometimes wrapped right round the stalk.  Broad-leaved Dock, the commonest dock, has a square sort of base to the leaf, while Water Dock and Wood Dock 
(you can tell them apart because one lives in woods and the other in water) 
have leaves that taper down the stem.

This is only a rough guide and my drawing below is not botanically accurate but just an attempt to make the description clearer.