Once again, thanks to everyone, but...

PROBLEM SOLVED....!!!

It seems there's a limit on how much you can add to your Pose library in P5. I just shifted a few rarely used pose sets into a 'spares' folder and hey-ho - my new set works fine! I also found an old copy of P3d0 on my ancient XP laptop. According to the read-me, it only works on XP. I installed it on my PC (Windows 7) and it works great. All my awkward PNG 'grey guy' thumbs are now RSR and show properly...

Happy days.