Gainsborough Lever Door set - Door won't open

Hi All,
My toilet door knob decided to break yesterday night locking me in the toilet. The knob would turn but the latch would not retract. Thankfully my wife could open the door using the door knob on the outside.
Trying to fix it by disassembling the door knob from the inside...
Door knob type here: http://i.imgur.com/l8EuRH4.jpg
I disassembled the door knob: http://i.imgur.com/ASE9x5n.jpg
Removed the outer bezel: http://i.imgur.com/v7r3x6r.jpg
The middle square bit could rotate freely but the latch does not move at all with it. It works fine using the lever from the outside. I did not disassemble it any further as there were no other screws to unscrew lol.
Does anyone know what is wrong with the door knob? I'd like to try fixing it before I call in the locksmith who will probably charge me an arm and a leg. Any ideas folks?
Cheers,
Vet


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