Update - I noticed the footwell in the rear drivers was slightly damp when hoovering last week so I checked all around it and decided to strip to door panel to check the membrane as suggested above,the membrane was badly torn and not attached at the bottom and the bottom of the door had a few mm of water in it. I dried the door and repaired and reattached the membrane and thought I was 'home and dry' unfortunately not!!! It's still exactly the same so next weekend all door panels are coming off and the interiors coming out so I can get to the bottom of it. It's driving me insane and putting me off the car if you know what I mean.
I have checked the rear light seals and they're good, I've also changed the pollen filter last week when I serviced it and it all seemed ok there too.
Will update you all ASAP. Even if it ends up in a for sale thread lol