@MrMurder - Its a screw that would be at the very front of the shell. However all the others are present and there's no way you can pull it apart.
@Duncan mctiernan - Yes I will accept that offer! pm on way.
To swap the back plate....its straight forwards...undo all the screws and pull shell apart. The back plate slides out. As far as usability goes...with all the screw in (excluding the missing front one) you will not be able to separate the shell so its all ok.