Many thanks chaps.

I checked my new evaluation version of P3d0 and the 'convert' tabs are all greyed out.

HOWEVER... I did a bit of brain-scratching and remembered I had an ancient version on my long-unused Windows XP laptop. I dug it out, plugged it in and fired it up. Copied the entire P3d0 folder onto a memory stick, whacked it in my PC and it works perfectly. ALL my awkward 'little grey guy' PNG's are now RSR and everything works...

I'll look in to getting a much newer version of Poser though. I just don't want to lose my HUGE amount of content on P5...