CHKDSK /F corrected the file system issue I was having, it repaired the file allocation tables and I could then access the directory structure on the drive. What I then had to do was enter each directory manually in file explorer and then wait a few minutes (it would appear to hang) depending on the size of the files present for it to "rediscover" everything.
I use it for Plex media serving but until I manually entered each directory individually and waited for it to bring the files up in file explorer Plex wouldn't respond to any media I tried to play. I'm still going through some of the directories now. I've lost a couple of episodes for one or two shows, but they will be easy enough to replace.
Touch wood though apart from those one or two lost episodes everything now appears to be working correctly.
I thought it worth noting that it did appear to hang in file explorer when entering each of the directories for the first time, I gave up on the first occasion and was getting ready to trash the drive, but luckily I tried again, and waited for it to find the files. I'm not really sure what it was doing, but it was almost like Cloud Drive was rediscovering and re-indexing the files as there was a lot of download activity while file explorer appeared to hang.
I hope this is helpful for somebody, and maybe you (and me) in the future if it happens again