Thanks for the advice!
Unfortunately the database rebuild didn’t reveal any additional files. I also attempted to run both a short and full diagnostic, both of which stalled at 90% (which seems to be quite a common problem from what I can find online).
I opened up WD QuickView to attempt to rebuild the database again and thought I’d reboot the drive just in case not completing the diagnostic test might have left it in an unstable state. So I shut down the drive (under Setting>Utilities) and once it had powered down unplugged the powere lead and plugged it back in again. On doing this the drive started a rescan which revealed more files, but stalled on one folder - I removed this folder and repeated the process and hey presto - all files and folders on the drive are visible when connected through Twonky!
Hopefully this can help someone else.
