Morris Minor Owners Club

Try Lemass Automotive in Chalfont St Peter. He does quite a bit of varied work and although the website shows some of his bigger flashier projects, suggest you give them a call - speak to Tom. I've seen his work where he repaired and resprayed his old 964 carerra 4 - tell him Blown Away from 911uk s...

I'm no expert but have heard that it has something to do with how far the end of the exhaust pipe sits from the back bumper.
It could relate to pressure differentials around the rear of the car.

Rob.