I have my pi nicely configured using the original release of volumio, however I’d like to update it without wiping out my customisations (for example the BBC update script.)

Is that possible from the .img file?

you need to save 2 files /var/www/db
and the mpd database at /var/lib/mpd/tag_cache

save them on your pc, then reflash the image and copy them back giving them 777 permissions

this should be enough…

Why 777? aren’t they read only by mpd?