I managed to get a small mirror down this valley of mystery and can see that there is a hole on the inner-most wall of this section (where the red arrows are pointing). I don't think that cutting the outer sill off is enough to get to this section is it? Is there another section to cut through to get to this inner wall?

I am stuck with how to do this part!
I have studied as many pictures as I can find of this structure in the passenger side rear sill area as I can find but I can't figure out what should be there and what shouldn't.
I have marked the attached picture with red lines and short green ones .
I can see how I need to cut the rotten left hand side of this bracket (for want of a better term) and weld a new piece in, linking the bracket to the inner sill / arch area. Where I want to weld it to seems to be filled with yellow seam sealer. It would be easier to get at and weld if I carried on round the corner so to speak past the seam sealer, but I am guessing I should scrape out all that yellow stuff first and then sandwich my bracket repair piece in to the gap and then re-seal it.
Is that correct or is there a better way?
also, should the "bracket" once repaired touch the inner sill end wall that it it at right angles to, or should there be a gap?
Any ideas guys?