Spent the day stripping the back of the car, in readiness for sorting out the welding repair. And it's actually quite a big hole. So, I don't think I'm going to attempt it myself, I think I'll give Jacks Hill Motors a ring and see if they can do it as well as the MoT retest.
Pics in the extended bit...
Yes, that's the inner sill you can see 
And side on, you really can see just how much has rotted away
And from inside the wheel arch, not as much rot, fortunately, as the repair panel I've got is for the outer part only.
