I don't know if hormone treated (HT) rapeseed (called Canola here in the U.S.) has been approved for use in Britain, but if it has, the farmer's must take extraordinary caution to prevent feral or volunteer reproduction of the HT treated plants. That includes treating the stubble fields after harvest, especially if the farmer is using low/no till techniques. The treatment can include application of pre-emergence chemicals... The pre-emergence chemicals are failry benign, but they are ( at least here in the U.S.) liquid applied rather than dry powder.
One poster suggested it may be lime. Lime or other soil treatment /remidification techniques are not uncommon here in the western U.S. where soil pH requires such treatment so that may be as accurate as any quess can be...