Water Leak in boot of 123

[Ian Waterhouse]

Every time it rains I collect a lot of water in the boot of my 123.  I have very little doubt that the water is coming through a rot hole under the rear boot opening lip, probably under a drain from the sun-roof.

I can weld, but I have no idea how I can get at the internal area to gain access for welding in new steel.. Can anyone who has done this please advise.  thank you.