It’s the inner wing, by the suspension turret. It seems the area has been kept damp by the stock airbox that would be there, and with a cone filter now fitted the damage has become evident.
I think it’s an outer wing off, cut and weld job. Ironically, the rear arches and sills are solid!
-be careful strapping AFM up like that, if the cable tie stretches or snaps the wire to the AFM is close to the manifold. I made a little metal bracket that goes from suspension turret nut to AFM whn I had my eBay sourced cone filter. Apologies if it’s only a temp fix and I appear nosy.
Thanks for the advice. It was fitted like that my a previous owner, but when I have new metal put in to the inner wing I will have a bracket made up to support the AFM then. It does look a bit half arsed how it is!