After a decent clean with Autosol , wipe over with Isopropyl alcohol I brushed some silver Hammerite Smooth finish onto the pitted area, left a few minutes and wiped off the excess.
The one on the right has been painted. There is a slight difference where the paint has got into the pitted area but it’s negligible, I struggled to see the difference

Having had a feel around the it seams the tips are tack welded in more than one area. Unless anyone knows something contrary it looks as though we will just have to live with the damage , changing the whole back box looks like the only way to replace the exhaust tips which I would imagine is going to be EXPENSIVE ☹