Both sides had a right hand thread. Lacking time and "gung-ho" I didn't pull down the caliper any further.
The rebuild kits in Peugeot/Citroën bags are actually TRW kits. Would be nice to work out what other seals are needed to do a full rebuild like the exchange units sold in Europe.
Peugeot want over £170 for a new master cylinder here; refurbing the existing one is possibly next on my radar...