Ok, you seem to have taken a 2 photos of the gearbox, wheres the rest of the rod?
Is it a 3 piece system or has the gearbox just fell off?
My guess is its a MILA locking mech but being 2001 highly likely is now obsolete.
Give these people a call.....
http://www.mdgrepairs.co.uk/
Ask for James and tell him Tom from the Remedial Company recommended you, have some measurements ready like the total length of the rod, the backset (which is 22mm), how many mushrooms there are in total and whether its a shootbolt mech or just a standard one.
Also looking at the state of the gearbox id say its failed because it was stiff to lock, correct?