I queried this to Tama Software (the developers of Pepakura Viewer/Designer) who came back with the following info. I thought I had the model I was working on centered but looking into it more I did not. With Blender, in edit mode with all selected, pressing 'n' will bring up the transform properties window in which the center-point for the selected can be modified. Assuming the model is symmetrical along the YZ axis, the median X coordinate needs to be set at 0.000. Once this is done the mirror point function will work when the model is imported into Pepakura Designer (provided that the front is correctly specified when importing it).