Premacy / Mazda5 Water Leak

I have a 2007 Premacy 7 seater, great little car but every time it rains water leaks through the rear door and pools in the tool tray and the spare tyre well.

I have taken the plastic cover off the back door and can see evidence of water all through the place. I can't go any further without removing the whole door and fully dismantling all its components. Has anyone had similar issues and if you did, did you find a solution?