When I had my Zed it had a few small bubbles on the drivers side rear arch so I took it to a local body shop.
They quoted me for some other spray jobs at the same time (respray front bumper & roof) but I believe it worked out around £250-300 for the rear quarter (repair & blend in with side/rear bumper).
I had the work done and it looked great for all of about 2 weeks before bubbles started reappearing, ...understandably pi**ed off I went back and complained. They redid the work this time taking pictures along the way showing me everything that they did. On their second attempt at fixing it they cut out a fairly large triangle shape piece of metal, welded in a replacement piece of new metal, filled, smoothed, primed, base coat & lacquered.
That's how it should have been done in the first place I thought. It only lasted for about 6 months though before the rust bubbles started coming back again!!
In the end I left it as the bubbles were very minor & with selling the car on the horizon at the time I didn't fancy plowing any more money into it.
Not much help I know Adam but just more of a cautionary tale. If you are going to try & get it fixed I'd consider getting a proper pair of new rear quarters otherwise you may end up paying more money every 6 months or so just to repair the same problem again & again.