Thank you for the very full reply.
I've had a lot of trouble (as yet unresolved) with my iTunes Media folder reverting to my Diskstation as soon as the Diskstation is online. The result of my messing about to prevent this from happening has been lots of efforts at consolidation and duplicate finding. I don't profess to be an expert in all of this but the result has been quite a few missing entries and files. For example, the other day I found a CD of Beethoven's Violin Sonatas, lost tracks 6 to 11 both as missing iTunes entries and mp3 files. I copied the mp3 files over from my Diskstation (without iTunes running), unplugged the Diskstation from the network, and then imported the files.
(FYI: There are no "orphaned" iTunes entries - i.e. missing mp3 files but with an entry in the iTunes database.)
The reason for my post was that I have over 2,700 albums in iTunes and manually going through each one to check if all the individual tracks are there doesn't fill me with excitement. A handful of albums have never had all the tracks - where I have only downloaded a specific "song". The vast majority though are ripped from CD's in their entirety.
I think I may resort to exporting (copy and paste the song info) into Excel and then run some magic to tell me which albums have missing tracks.
I'll keep you posted.