I had a Rover 75 with this problem. It turned out to be a cabin air exhaust vent that had popped out of the aperture in the bodywork, leaving a gap that water could pour through. It was hidden behind the boot trim on the inside, and behind the rear bumper on the outside. I suspect it was dislodged when someone took the bumper off to fit a towbar. All I had to do was snap it back into place and that solved the problem. Water ran across the boot floor and down into the rear foot wells. I had to take the carpets out to dry off as they were soaked through.