Once done, I put the plug back in and ran the angle grinder with wire brush over it to remove excess rust, then slapped a load of Hammerite in the iron casting and brace rods (which I didn't bother taking off).
It's not perfect but will protect the casing from further corrosion.
By the way, I took a shot of the diff brace. Here we see it is a little more than a piece of angle iron. There's a spacer braised onto it and a cutout for the breather plug, plus an indentation that goes round the rear of the casing.
Inspecting the diff, I can see that it's been apart before. There is evidence of a gasket sealing compound around the rim of the casing join. I can feel the smallest amount of play in the diff action but once this is taken up the action is smooth. I would like to think it's been properly overhauled at some point, though I've no documentary evidence.
So, all ready to do the seals, then. I scraped the excess grease from the underside of the car where it had been spraying out of the pinion shaft and packed up. Total elapsed time, about 9 hours.
A quick question for those of you with Sue Miller's CV joints. My output shafts are "fail safe" types, with a pin coming out of the centre of each of the three legged spiders. It's the same at the wheel axle. Do I need to modify these, or will the CV joints fit as is?