Thanks folks.
So answering the questions, yep have poked and prodded, the inner arch is actually pretty good other than the visible crusty bits, the rest is solid, so it's purely the front lower and rear parts of the arch that I need to repair. That front bit it really only the lip, the rest is good.
I think I'm going to get an arch, but am going to cheap out and try the 440 rear arch as I'm only really after the curves/bits and will weld the rest in so I think it's worth a shot at a fraction of the cost.
I can then use the lip all round and the rear part to make the section I need. Then I'll pre-fab the sill 'rectangle' that I need to patch you can see in lower right of photo.
Annoyingly all on ebay is LEFT rear, no right which is weird!
Hoping that's a plan. It's never going to be a showroom condition but so long as I can patch it and get it to MOT state I'll be a very happy person indeed.
If you want a closer look I've posted a youtube video on this bit and attacking it with a flappy wheel to get the surface off and see how many holes appear.
Again, comments appreciated. Should I be bothering keep popping into my head. The engine, gearbox, etc are all sound, it's kinda annoying the rear is rusted as an MOT failure as otherwise I reckon I could get this through an MOT with just new shocks, etc.
Video if you're interested
https://youtu.be/LwklXO7t9pI