Okay, figured it out. I think I know what happened... it's a bit long winded unfortunately:
After I originally ripped the album I wanted to compare the file sizes between the versions ripped with MusicBee and a version ripped with iTunes. So, I renamed the MusicBee album in the artist folder by adding and underscore to the end (I don't think I had added the album to MusicBee yet at this stage) and then ripped one track in iTunes to compare file size in Windows Explorer. Anyway I deleted the iTunes one and removed the underscore from the MusicBee folder and re-ripped the album again in MusicBee. I then tried to add it with no luck as mentioned above. This morning I decided to try and add the underscore back into the album name just for the hell of it and it worked, adding all 8 tracks, when re-adding to MusicBee. I removed the underscore and same nonsense as mentioned in my post above
So to solve this stupidity, I then deleted my MusicBee library and re-added everything and it worked! So somewhere along the line MusicBee didn't like me renaming stuff outside even though the album hadn't been added yet.... not sure if it was picking up iTunes' library of something and there was a conflict?