2003 Ford Fiesta I have no wheel nut key! Help!

Hello, I have a ford fiesta lx 53plate and recently my tire popped. As I was trying to replace it I realised that 1 of the nut fings had a hexagon looking shape in it.I heard that apparently I needed my locking wheel nut key which sadly seeing as I bought the car private, theirs always things missing.I wos told to go ford mechanics n buy a master key, other sources tell me 2 smash in the thing! Seeing as im a sane human being ild really be greatful if you could tell me the correct prrocedure 4 this situation, and if I do have to go to the mechanic, how much would it cost?