Could be the hills Lee. Most of the UK is hilly, apart from the East anglia bit where John and Chris live, so stability would be an issue. Fields are also smaller in general and usually accessed by the local road network, so again, stability at road speeds could be an issue. I must admit to not quite getting the concept though, so must be missing something. Surely putting the front wheels inside the rear wheels like that causes more damage to the crop? If they are in line with the big driving wheels at the back then damage made by the rear wheels is the limit?