Ok I made a rear undercover panel which I hope will stop this "parachuting" effect people are talking about. It's not going to be to everybody's taste but I might make some alterations if it doesn't look right when it's fitted.
I cut some holes in the middle outlet to release some of the air coming through the transmission tunnel. I'm thinking of adding a mesh grill to this outlet also to give it a bit of character.
Hoping it will look better when it's black and fitted to car.