Disco 1 How to treat & fix this little bit of tinworm???

Got cracking on the Discovery again after a 6 month lay-off due to a knackered back, so this needs curing its a 2 inch by 1 inch hole which is on the near-side rear wheel-arch at the bottom where the chassis leg meets & attaches to the sill. The sill bracket is ok, the chassis leg is fine too but its the bottom bit (the flimsy bit) of the arch that's rotten. Looking at it it seems the bracket that is welded to the sill is also (or was also) welded to the bit of the wheel-arch that is now missing in some kind of sandwich. Would it be just the case of welding a new piece of plate in to fit or will the patch need to be welded to the sill bracket also???

I'm also conscious of the rubber mounts & don't want to melt them

I had same issue , I cut out the rust and plated over the hole only . .

I was thinking of just doing that, but looking at it, it looks like its attached to the body mounting? So I if I cut out the crud would I not need to somehow weld it back to it???