There are some difficult challenges for the casual nature enthusiast where botanical knowledge is, shall we say, sketchy! That is the case with me, I can cope with some of the dandelion/hawkweed issues but then species like the cornsalads are beyond me. I label all cornsalads I find as common cornsalad (Valerianella locusta) whereas, in reality, it is possible that they could be one of the others.
Cornsalads are members of the Valerian family and are generally small plants each with small clusters of blue or violet coloured flowers. There are five similar species and there are differences in structure but examination of the seeds (or fruits) is the only real way to come up with a certain identification. Common cornsalad is, indeed, by far the most common and then narrow-fruited, keeled-fruited, broad-fruited and hairy-fruited can all be found in Dorset although broad-fruited I believe is more likely to be found in Hampshire.
All of these species grow on arable land as well as places where the soil is bare and where competition is minimal. It grows readily in the gutters of the roads around our little housing estate where there is very little soil. The leaves are edible and hence, this was once called Lamb's lettuce.