Rover owners: I found the leak!!!!

At last after two winters (possibly more) of having a leaky passenger footwell I have today found the problem.

I pulled the wing off again and decided to start from the bottom and work my way up with the garden hose. It looks like I missed a seam that was behind the outer skin of the wheel arch which can be accessed from the small 2p sized hole.

The issue I have is I can't physically get to the seam in order to repair it. Any ideas on how I can access it? I don't really want to make the hose bigger and I am unsure whether coating the sod in cavity wax will work.

Here are some better shots of the other side (well as best I could)