In my experience corn spurrey (Spergula arvensis) is not a common weed of cultivation in Dorset. This is a plant that does not like lime soils and much of Dorset's arable farmland is up on the downs, the lowland acid soils tend to be heath and acid grazing, not much in the way of arable crops are grown on acid soils.
Corn spurrey is a low growing, rather straggly plant but it has attractive little white flowers with the green sepals clearly visible behind the petals. It does not have much in the way of leaves, just whorls of thin needle-like leaves. It is a member of the chickweed family and has much in common with its relatives.
As with many weeds it seems, it is a strange combination of crop weed and food crop! Where it occurs it can be a major problem and as its common name implies it like to grow where corn grows. However in some places it is used as a culinary herb and cultivated for that purpose. It is quite harmless having no poisonous ingredients and yet it can 'poison' a crop if it gets established.
like many farm weeds it has several local names such as devil's gut because it is stringy and sticky! It is also known as pick purse because of the shape its seed pods and starwort because of its star shaped flowers. The name farmer's ruin seems a bit unfair, it is not that bad!