There was a couple of issues in the character rig that were confusing the issue, but once I'd fixed those separate issues, this problem became quite clear. So, just in case anyone else makes the same mistake that I did, all you have to do is just make sure to import the Xrefs with 'Name Space' set to 'None'.
By default, the pose Library will only recognise exact naming conventions, so perform these steps in order to ensure that your naming conventions are identical when you import the Xref.
