Here is some advice from another site , and an alternative quick repair from E-bay .
" Hi ya, i work in a bodyshop, its common. If having it repaired, make sure the filler cap assembly is removed. its quite easy, take off the fuel filler cap & you will see a phillips screw just of top centre, unscrew it & then rotate the complete black plastic assembly (with the fuel filler flap attached) anti clock wise & it will come off the car, this way you dont have to remove the fuel tank. I tend to use a die grinder to grind out rust/ corrosion then treat with Q-rust. skim it up with top stop. flat & then 2k prime. Prep & paint the quarter panel out. I always remove the rear quarter window too as its only 4 screws & it makes a much neater job.
I always smear grease on the apature before refitting so as to repel any water spray coming up from the N/S/R wheel. "
FORD KA FUEL CAP - RUST SCREENING / REPAIR RING - 215mm | eBay
Moved post to correct section .