Where there are bare patches of ground on the heath, often alongside footpaths, or sparsely grassy places then you may well find sheep's sorrel (Rumex acetosella); it certainly thrives on acid soils. It is a common plant and where it occurs it is usually quite well established.
Sheep's sorrel is a member of the dock family and its maroon flowers quickly give way to red seeds and the whole plant takes on a reddish-brown hue giving the appearance of being withered and finished when actually it is still quite active. The leaves are shaped like an arrowhead and I recall, as a lad, we used to bite them to release a bitter taste, we called it the vinegar plant! It is apparently also known as sour weed and the taste comes from oxalic acid. It is a small plant, rarely growing to no more than a few inches tall. It is much smaller than its similar relative, common sorrel.
It seems to be well thought of as a medicinal herb and is used for anti-cancer therapy, as an anti-inflammatory agent, an anti-bacterial agent and immune system booster, an all round good egg it seems!Sheeps Sorrel: the sour weed