Thanks Kirk.
I suspected this might be a problem that was not created by PM6+.
So, I opened the 'locked' .NEF files in Nikon Capture where I was then able to unlock them, and to be safe rename them as well.
Then went back to PM6+ and using the Manage Catalog command, and Maintenance command have re-indexed the catalog which has resolved the problem completely, I can now add keywords rating etc. which I could not do before.
All seems to be working nicely now - many thanks for the very quick help.