SUCCESS !!!
I bench bled a brand new Girling 3/4" MC, plugged both holes, fitted the MC, pressurised the reservoir, and bled the air out from the pipe going into the side of the servo.
I now have a solid brake pedal.
The faults turned out to be:-
4 rubber brake hoses that had closed up, and needed replacing.
1 brake drum wheel cylinder that would not pass fluid through to the bleed nipple.
1 faulty brand new master cylinder that had been sitting around dry for a few years.
Cost of fixing, about £28 for hoses, £12 for wheel cylinder, £30 for a master cylinder and about 4 litres of brake fluid which managed to leak everywhere.
Thanks for your assistance. Discussing a difficult problem with others gets there in the end.