Update on the whole rust thing.
Just before Donny I took it back to the bodyshop and had it patched up so it wouldn't worsen, which the owner kindly sorted for me. I provided some rust killing stuff which showed up a potential problem area at a weld point. So it may be coming from inside a weld point, which is why it didn't go away first time and came back so quickly. And also why it's probably not treatable simply by removal and sealing, which was the best option at the time in his opinion, as it appeared to be simple small chips.
But we're not 100% sure - need to check top of arch first. So the summer project is me taking the wheels off on fine weekends, rubbing the lip and top of the arch right back, allowing rust killer to soak into weld points and fully sealing it, one at a time. Paint should be OK to match as it can be masked off with the join at the outside of the arch which makes my life a lot easier as I would otherwise mess it up
If it comes back quickly again, I know someone who is a metal worker who should be able to cut the arches back towards the bodywork and seal them up. Then if that doesn't work, we resort to new arches and big money!
So thanks to everyone who gave advice - again I wasn't looking to cause anyone problems, simply for advice as I like to be fore-armed when trying to resolve things. As it turns out, I won't have to unless we get to the point at the end of the second paragraph. And if it goes wrong this time, I have no-one but myself to blame
I still like to get advice and discussion over products and services - as an expert in one field, I appreciate how little the layman knows about other fields of expertise!