Many thanks for the suggestion.
Obviously the lack of an app within Sonos isn't ideal, but accessing the service via Airplay could be viable, and I gather there's another workaround: anything saved as a playlist in Apple Music Classical will automatically appear in the Apple Music app in Sonos. Let's hope Apple decide to create an app for Sonos.
As far as the metadata's concerned, based on a quick test I can see Apple Music Classical is massively better than other classical streaming services. Search for "J S Bach" in Apple Music Classical and you do actually get all the music by "J S Bach", "Johann Sebastian Bach", "Bach, J S" and "Bach, Johann Sebastian" (and no doubt all the incorrectly spelled variants) ALL IN ONE PLACE. This isn't how things used to be and is hugely better!
The problem is when you then want to browse J S Bach's compositions, they're all listed in what seems an entirely random order -- all 1,224 of them. Not in alphabetical order, not in order of BWV number, and not in subgenres (which is what I'd prefer). Which makes browsing a real PITA.
But it's still a pretty good effort compared to other services, and I can see it being usable, if frustrating.
So it turns out that AirPlay doesn't work with my (rather old) Sonos kit -- a motley collection of ZP120, ZP90 and Connect gen 1 boxes. I was expecting (irrationally) that AirPlay would be fine because Spotify Connect works with the old Sonos kit, but not so.
So the only way to use Apple Music Classical with my older Sonos gear is to create playlists in Apple Music Classical and then play these using the Apple Music app in the Sonos controller. That's too clunky a workaround for me.
I've also explored the Presto Music streaming service. It has a smaller list Apple Music Classical, but a better search function. (For instance if you search for a specific recording by giving the composer name, piece name and conductor name, Presto will give you a search result consisting of the desired recording only, whereas in Apple Music Classical the desired recording could be halfway down a long list of other recordings.)
Another advantage of the Presto streaming service is that it contains details of reviews (e.g. from The Gramophone or from BBC Radio 3's Record Review). This would be a big plus for me, because The Gramophone and BBC3 RR are my first ports of call for finding new recordings.
Sadly there's no way to play Presto Music on Sonos devices other than via AirPlay, so it's a non-starter for me. I gather Presto Music are working on integrating their service into the Sonos controller app, and I have a hunch they'll get this done before Apple do.
Another potential fly in the ointment with Presto is that it *seems* to default to hi-res versions (I still have to check whether this is in fact the case), and hi-res files won't play on Sonos.