Got exactly the same problem, on mine same place but opposite side.
I went to a body shop that I've used before and trusted they cut it out and welded new metal in but it came back worse.
So I Had a little poke around the rear arches and couldn't really see a problem from there. But then I started realising moisture in the boot area of my zed (boot glass would always have condensation on it) and after stripping the back out I found that along the rust bubbled side there seemed to be a fair amount of moisture leading me to think it may be a leaky seal or something, causing it to bubble up from the inside?
I haven't got any further than this at the moment but just wondering if anyone else has the same?