Both librarys the same now. I started to try and compare MB libraries on both PC screens to find the final missing 13, then had a brainwave and ran manage duplicates on 1st PC, even though there should've been no duplicates, it found 13!
So, dragging the library folder into the inbox was more successful in replacing missing files than rescanning both 1st and 2nd PC library folders over and over. There's still 5 missing files in playlists, but out of 4279, that's not bad, they'll be easy to find another day.
Next step now is to add new files to library in 1st PC and then synch folder and playlists to 2nd PC.
To be advised