Thanks, offending area above. Hole is at the bottom just in front of the wire brush.
I've already cleaned up the rust using anglegrinder and wire cupbrush (fun that was!), then cleaned with strong detergent followed by por-15 metalready (zinc phosphate/phosphoric acid?) then after several good rinses and a really good dry (fan heater)put 1 coat of interprotect. Seems to be adhering well but you can see some of the existing paint
where there was no rust bleeding through.