I continued working on the rear battery box and decided that I would remove the front section as well, preserving the upper tabs in place. Progress so far today
Top left tab exposed, battery box metal remains above

View from the wheelarch to the tail light

What I cut out today

And a first trial fit for the replacement panel

Are the (fillet ? ) pieces available that go vertically from the wing to the battery box or do I just have to make one from sheet steel.
Similarly, for the horizontal surface.
I did remove the rear light unit to look at the other side, it seems there should be a horizontal piece, but as this has already been "repaired" (it looks OK from the inner wheelarch / underside) I don't know what the correct construction should be. The orange is the inside of the sliding door as the rear wheelarch has holes in it after a tickle from the grinder.

Looks like I now have both battery boxes to do, but the left hand side will wait until the wheelarch etc has been removed. Much easier access.