Well, it's been longer than expected, but I've finally got my junction box sorted out. Those of you with good memories will remember that I posted this:
After re-arranging the earths into some sort of sanity:
So, after a lot of checking I discovered that half the cables weren't connected to anything. I also found another junction box not passing through the earth - not so bad when feeding a transformer for 12V lights, but I wanted to fit mains ones. I also found another ceiling rose being used as a junction box, and a 13A socket fed from the lighting circuit - though on a shelf, so it looks like it was intended for a light. No label, though.
Having got rid of the redundent cables, removed the surplus junction boxes, and replaced both the dining room and bathroom lights, it now looks like this:
And with the lids on:
Everything labelled, no redundant cables, the earths connected to proper terminals (and sleeved). There's still one flaw - there's no sign of equipotential bonding in the bathroom. There's an RCD on the shower circuit, though, and the shock risk from the lights should be pretty low since they're out of reach without standing on something, so it can wait for now.
Now to find out what's in the other loft...
Mike