I've never had to remove a rear arm before, how do I remove the brake pipes? Undo that little nut on the axle tube and take the pipe off the three way union and withdraw the whole thing or do I unclip it from the arm and take it off of the wheel cylinder instead? I intend to just clean and paint the bits and stick them back on. Is it worth buying new rear brake pipes when there's bugger all wrong with the old ones apart from being old and grotty?

Not sure if it's the best or proper way but i usually detach them at the cylinder and bend the brake line a bit to get the mounting plate trough the arm.
If there is any rust pitting on the lines, replace them, preferably with Cunifer (copper nickel iron)
If you go the cleaning and painting route, i would suggest to completely remove them.

its a sod of a job but doable
best to leave them be if poss

So how would you check that the axle bearings aren't on the way out?
Replacing those is even less fun than replacing rear brake pipes with the body on.
Considering that a pair of cupronickel pipes are about £50 and are likely to last the life of the car, I reckon that it's well worth replacing any steel ones which have started to rust.