Thanks for your advice.
Do you have any idea, though, why an Album with the Compilation property set does not show up in the Compilations list in Artists view? (I have some albums with no Album Artist that do appear here (and others that don't), so there seems to be some kind of bug.)
If I turn off group compilations in the Show View Options preference it means that the only way I can find the album (in this view) is by scrolling through the list of all artists.
Ideally, I would like to see the individual tracks appear under their artist (as I believe is supposed to happen) with the whole album in Compilations.
I have discovered what's happened: and it's worse than I thought.
A huge proportion of my compilation albums seem to have lost their individual track Artist data. (The only cause of this is likely to be iTunes Match.) Those are the ones that, unsurprisingly, show "Unknown Artist".
Now I have a new problem: I have added back the artist data to one compilation album. On the Mac that I did this on, the Album view now shows "Various Artists". However, on another Mac that's subscribed to the iTunes Match account, the Album view show the name of the artist of the first track of the compilation.
One thing that does now work: once I have been through a compilation and re-added artist data to each track, it now appears in the Compilations 'artist' in the Artists view. (Others which seem to have their artist data in tact, do not, but that's now the least of my problems.
I DO have backups, but I am not sure how long ago this corruption occurred so I have some searching to do!