Problem solved…
I was looking at /var/log/samba/log.smbd and found lots of reports regarding widelinks and being unable to load a module
[2022/10/13 22:59:13.250524, 0] …/…/source3/smbd/vfs.c:379(smbd_vfs_init)
smbd_vfs_init: widelinks enabled and vfs_init_custom failed for vfs_widelinks module
[2022/10/13 22:59:13.250616, 0] …/…/source3/smbd/service.c:636(make_connection_snum)
make_connection_snum: vfs_init failed for service osmc
[2022/10/13 22:59:13.254588, 0] …/…/lib/util/modules.c:49(load_module)
Error loading module ‘/usr/lib/arm-linux-gnueabihf/samba/vfs/widelinks.so’: /usr/lib/arm-linux-gnueabihf/samba/vfs/widelinks.so: cannot open shared object file: No such file or directory
[2022/10/13 22:59:13.254819, 0] …/…/source3/smbd/vfs.c:185(vfs_init_custom)
error probing vfs module ‘widelinks’: NT_STATUS_UNSUCCESSFUL
An internet search eventually found Samba: No shares available after bullseye update which discusses this.
It talks about disabling wide links (but I currently use them so don’t want to disable them).
Alternatively, post #14 suggested
sudo apt-get install samba-vfs-modules
which fixed the problem for me.