Tried Daniel's advice to valebe in another post about artwork not showing up on his infotainment system. Although my problem is on the B2 I thought I'd give it a go.
In one of the offending albums on NAS I deleted the coverart.
Ran Scan Disc and confirmed on both the UI and NAS that the artwork had gone.
Replaced it with one from another album I know the coverart is displayed, again via NAS.
Ran Scan Disc again and checked the UI.
New art work showed in the RH Album window ok and the track played fine, but again no show in the Now Playing window.
Time to try something else. All the offending albums were downloaded from Bandcamp. I had downloaded them as mp3 and they came as a zip folder of the album containing the tracks and the coverart. Unzipped by dragging and dropping into a new album folder. I put that into an Artist folder and the then transferred that folder via NAS to the B2. Everything worked fine except for the coverart not showing in the Now Playing window.
Bandcamp give you a choice of formats you can download, so I thought I'd delete the offending albums and re-download as better quality FLAC, but this time I decided to discard their higher resolution coverart and get it manually.
Used NAS to delete the albums from the B2.
Ran Scan Disc and confirmed on both NAS and UI that they were gone.
Next downloaded the albums from Bandcamp as FLAC.
Opened Album Zip folder and dragged and dropped the FLAC tracks to a new album folder. Left the coverart file in the zip folder.
Went back to the Bandcamp web page and managed to capture a lower resolution image of the album coverart, and saved it to my desktop as a jpeg.
Moved this jpeg into the new album folder and put the album folder into an Artist folder.
Using NAS transferred the artist folder to the B2 drive. Ran Scan Disc. Crossed my fingers and checked the UI. Eureka! Coverart in both windows as it'ssupposed the be, and the tracks play too.
Can't say for sure what was blocking the the coverart showing in the Now Playing window, but a complete album removal from the B2, an album re-download from Bandcamp replacing their the high res coverart with a lower res image before transferring to the B2 seems to have done the trick. A lot of mucking about, but I'll know what to do in future. Also I upgraded the albums to FLAC. Don't think that has anything to do with it, but I should have gone for FLAC first time round. Happy Bunny now. Don't know if the original High res coverart had triggered something on the B2 that the deletion of it and replacing it with lower res image within the album wouldn't allow. Took the deletion of the whole album and replacing it with the new one, with the lower res coverart already in place. Anyway, sorted. Phew!