Frank, I have the same reaction to the smell of oilseed rape - only passed one field today (not too much in the New Forest) and that had me reaching for the window buttons.
Much MUCH worse, though, is Privet flowers. They aren't in huge fields (thank goodness) but you have to pass garden hedges when you want to walk about the area. I hate the beginning of July.
New Forest was gorgeous in the sunshine.