Sussed it!
Not sure whether "Other Video" is a default Role in FCPX - I suspect it's not. Anyway, I use Roles and the Timeline Index to shrink connected clips and titles in the timeline to save space - then when I want to edit them, if I want to see the thumbnail, I use the little arrow in the Timeline Index to enlarge these clips and titles.
Well, for some reason, FCPX had assigned the "Other Video" role to my multicam clips - and because I'd set these to "minimise" the thumbnails disappeared.
Slightly obscure, but thanks to Luis Sequeira1 I found it. I copied a project (that didn't have the problem) into my Library, and the project was still OK, so this suggested the Library was OK.
Then I created a new project and pasted all the clips from the problem project into it, and of course, ALL the clips, including titles were huge - but the thumbnails were there. When I clicked the arrow to minimise the "unwanted" clips, all my Multicam thumbnails disappeared!

So thanks, Luis, you didn't actually pinpoint the problem, but you guided me in the right direction!