Aha, a quick ddg found the problem and a solution, it's the id3 tag - at first it seemed onlu 00 - 99 was allowed, my files where 0001, 0002, 003 and so on.
The solutuion is to install Easytag
Code:
apt-cache search mp3 | grep tags
sudo apt-get update
sudo apt-get install easytag
Okay the search line isn't strictly needed.
I installed Easytag, opened each audiobook folder and hey presto they each got updated as I moved to the next folder
All good now
SOLVED!