rear passenger door wont lock on central locking but dont have key fob only?

i have a 2001 (51 plate) mondeo ghia x and have lost the main central locking key (which if anyone has any idea also on how much these cost would be grateful) so am having to lock it via the drivers door when locking the door the locks make a noise but the drivers side rear door does not lock, any ideas as i am a women and dont know anything about cars only how to drive them and fill with fuel so dont wanna take it to garage and be ripped off!!!