Thanks for the message rayooo and it was a help - helpful to know that this setup should work (and indeed was working for others).
So... after a bit of head scratching I've now rejigged my network to see if I could improve things that way and it has helped
Previously the PC had been connected straight into my BT Hub and the NAS connected to a switch which connected to the Hub. I've now moved the PC connection to the switch too and MC seems much happier.
Looks as though the Hub being between the NAS and the PC was slowing things down significantly.
Tag updates when adding newly scanned artwork files for the last couple of CDs ripped have completed in between 16 & 28 seconds. Much better than before.
The Recent Albums view is now fully populated with artwork again.
Full screen viewing of artwork is also working again.
The mini thumbnails are back for most files when mousing over the list of tracks but only after I view the tracks in the Files view first - this seems to generate the mini thumbnails and populate the tag info somehow.
I still have some missing mini thumbnails in the Files view list but suspect that if I re-update the album art for these tracks it'll sort itself out.
The puzzle now is why MC17 would work OK with my old network configuration but MC18 wouldn't. The BT Hub was obviously preventing communication, handshaking or just causing timeouts between the MC18 software on the PC and the files stored on the NAS but for some reason MC17 wasn't affected by this.
Don't you just love computers