[humph]

As Laura says sounds like tramlining. This is where the car follows the contours of the road (ruts worn into the road surface). It is usually particularly bad under braking if the road you are travelling on is heavily trafficed and worn. Again as Laura says it usually gets worse with bigger/heavier/wider wheels and tyres. If your ONE had 15" wheels with non run flats, and the sidewalk has heavier 17" wheels and runflats then there will be some (minor) tramlining. That said yours sounds particularly bad. Get the service guys at your dealer to give it a test drive, see if they think there is a problem. Don't forget if you think they are fobbing you off you can always take it to another MINI dealer.