Same issue here, after a couple of goes my dealer eventually fitted a 2014 cap which has a substantial washer on the inside of the cap supporting the rubber washer. This allows the cap to be screwed down hard so no leaks, and the rubber washer is properly supported so it doesn't rip etc.
My advise would be to refer the issue to your dealer / morgan. If the washer tears and falls into the tank , it could block the exit with untold damage.
same with my car
