I reckon, if the bellows being cracked was the only problem, it just wouldn't unload, as it is the bellows extending that unseats the unloader valve, allowing the air to escape.
the air is escaping at the moment because I think I have failed to seat the valve properly, before I stripped the valve down, the unloader valve was most definitely stuck in the bore, it would not move at all. Now I've lightly cleaned the bore, the valve moves freely, as it should.
I couldn't get to work on the militant this weekend, as work got in the way :-( but for bedtime reading, I did notice that the manual does say that if the valve is not correctly seated air will escape, which is what I hope is the problem.
Even if at the end of the day it still didn't work, at least I've learnt a little more. The lads in the garage are completely baffled by the whole concept of repairing something, as nowadays it's all about throwing away and fitting new.
The diagram HTML you sent me is almost the same, the only difference being that the Mk1 militant unloader valve also comprises a safety valve as well.