This works, for now.

My old wire brush failed me when I wanted to use it the other day, while I'm waiting for a new one to arrive a brain wave just struck me to try welding a nut over the failed thread section.

It works but I don't think it'll be long before it fails again, the extra length of the assembly will doubtless add enough extra stress to cause issues but we'll try to get another job out of it.
I was welding a bit blind too due to the nature of these modern automatic shades, I'll have to try and fit a battery holder into one so a measure of reliability can be gained.