In reply to LastBoyScout:
> Out of interest, what's your evidence for the leak being in the master cylinder?
I still need to be sure, but I think there were signs of oil around the lever / reservoir. Could have been left over from a messy bleed so I'll check before I try replacing anything.
> Failing that, I just take things apart carefully and take lots of photos of where each part goes and which way round.
Trouble is, I can't even see how to start! Nothing obvious to undo.