Thanks for the advice guys, much appreciated.
I took a video, which was the best way to get an image of the hole because of the light on the phone being on constantly. I'll put on the vid on my photobucket page and link on here. The bottom of the pillar below the door vent was wet so could door seals be a issue too? I have new ones to put on anyway.
I actually got inside the trunk
and had a good look along the inner wing towards the door vent, under where the rear window wraps around. Didnt see any rot there, but guess as there's a couple of bubbles in the paint near the seal, could be water getting in there.
My other issue is I dont have a garage and my storage for the winter has fallen through... so will only have weekends when I can get over to a mates and do stuff like taking the interior out to have a good look around. Its my daily driver at the minute because I just love driving it!!