Ok, done a bit more digging.
I copied some of the flac files to a Windows 10 PC also on the network.
These were visible and playable in BubbleUPnP, so the issue isn't with the files or the client (BubbleUPnP) but probably with the server (minidlna on OMV).
Do I need to do something to ensure that flac files are indexed?
Any suggestions gratefully received.