ive just done something similar.i couldnt unlock the back door the lock master lock had stuck at the bottom.the key and handle would not turn.(its a two man job)what you have to do is put wood under outside of door prise up use metal levers .get from B&Q prise open enough to knock lockmaster closed,its sounds barbaric ,it does do damage to outside plastic.door should open.take door handles both sides off, remove lockmaster strip in centre of door remove barrel lock .remove dodgy lock master lock by undoing screws along strip.then take thin metal rod off centre key lock it just slips off.slide the lockmaster strip back into position making sure the screws line up(dont put the barrel lock screw in leave to last.insert barrel lock slide in & out until key turns lockmaster tighten screw,replace door handles shut door ,door locks well done save ££££££££
the fish, October 2010