Thanks for clearing up my confusion, worrypascanada :) I have to admit I was startled to see the long list of files, because the version I have only has the 8 files I had listed before. Perhaps Andot, the developer, has since updated the phprpc version for this plugin.
I've just tested this 'new' version on a local 2.5.1 install (with default theme) and it appears to be working just fine.
Quite frankly, I am actually a little baffled, as I'm now starting to wonder if it's about permissions or clashes etc. Anyway, if you don't mind trying a slightly different solution, I've uploaded my (older) copy of PagePost and provided a download link below:
Hopefully you might find it useful. In the meantime... I'll keep digging :P