Well, just got the car back, and those rear arches are now looking magnificent with no rust!!!!! Cost a bit more than originally quoted, as I believe they cut a bit of the worst rust out and welded a plate in. Also I asked them to fit a replacement second hand front wing I'd bought to replace a crusty one currently on the car. After messing about with that, they said they were unable to fit it and get it to line up (see my other thread!!), so they put the old one back on.
But just fixing up the sanded down patches that I had treated with Bilthamber Hydrate 80 to prevent the rust from worsening, and pushing out a dent that was a bit higher up the panel, has just made the car look much better, and less "shed" like!!!!
I struggled to upload photos as the filesize was too big, but will try again later!!
Next thing I was hoping to get a quote on was some waxoyling