My last mot had an advisory "Suspension component mounting prescribed area is corroded but not considered excessive both rear outer sills (5.3.6 (a) (i)) ".
Is that inside the rear plastic wheel arch trim?
You are best taking it back to the most station you had it checked at and asking them to chalk the areas in question or show you then there is no issues about exactly what needs attention ( take a piece of white chalk with you )
Any decent people should be happy to show you , also if you are going to get it welded and are keeping the car longer term ask them if there are any other areas you may as well sort out at the same time sometimes they don't note corrosion on none suspension or safety areas but these patches may cause leaks into the car causing damp issues .
Take off the addition that is screwed with plastic nuts, buy anti-rust paint and a pot of antigravel, buy a brush to give the antigravel, which are cans to give with a paint compressor, but with a brush it works well, you sand the area, you paint with spray, and when it dries you paint with the brush, you mount the plastic addition and you forget about it