So here we have the reasoning It is broken and I now have a bag full of spares (Well that isn't supposed to happen!)
but this has lead to me taking it all apart and having a look inside (an exploded view of my mech!)
Jockey wheels and cages
Locking button, clutch assembly, clutch housing, parallelogram, return spring.
Parallelogram, hanger and parallelogram spring.
Roller bearing clutch.
Clutch explination: The clutch mech is actually a roller bearing clutch, due to the locking in one direction (before it gives way to movement) it acts as a damper (so when the chain is getting more tight) and is free to rotate in the other direction.
Any questions about how it goes back together are welcomed but I don't think it is going to live again (see the grinding wheel marks on the outer parallelogram).