Treat all rust once cleaned with two pack zinc rich epoxy,jutin clark or sigma are good quality paints.
I went to college with a guy who ran a FIAT 127.....It rather reminds me of an Engineer I worked with at GEC in 1976 who had a Austin Cambridge, every time it failed the MOT for rust he would bolt in a bit of angle iron then weld it up!
As you live in North Shropshire and of course if you haven't already done so. When I was living in the wilds just outside Kerry, I opened a cash only account with Boys & Boden where first of all I had cleared with them that I was about to start a major building project and gave them a full list of the types (but not quantities) of the woods, plywoods, adhesives etc. etc. I would be needing.So we took a run out to mid Wales