Looking back....it was always going to happen....
Sprayed it with a bit of plusgas......didn't wait any length of time, no preheating/freezing or whatever, just put on the snugly fitting 13mm socket, and give it a gentle try with the normal ratchet.
Obviously that wasn't going to work. Thats when I spotted the breakerbar...no point straining myself with the little rachet....on it went.
Plenty of leverage there....and......bingo the nipple's off sheared flat with the top of the caliper-the threaded section obviously didn't budge at all.
So I lost my sheared bleed nipple virginity today, surely there's none left now on the FCF

Regards Neil
Apoligies to FCF purists...it was on Luton's Finest not the Picasso!