These are the boot air vents, and if they are the same as every other MG Rover afterthought, they are sealed to the body with a "foam" gasket, and held in place by rivets.....

Normally these are ok, as they are not usually directly in the water line, being hidden by the bumper. However if for some reason your bumper might have been chinese, or not quite fittted 100%, I guess its more than likely it would let in water where it meets the quarter panel.