Walking through a field of unknown purple flowers

Nobody present on the walk could identify the crop, having decided it was not linseed, which has a similarly coloured flower. Does anybody know what it is?

Possibly lucerne, also known as alfalfa. This and other possibilities are discussed on a "Yahoo! answers" page (you may have to log in to find it); search for What is the purple flowered crop seen in British fields? That page gives it the name Melilota sativa but I think that this is a mistake for Medicago sativa - Leonard Will