Actually appears to be a couple of bugs here.
I've created a device to sync the MP3's as mentioned above.
The device sync's ok, and MP3's are all in the right place.
I then have an MP3 Library which finds all these MP3's no problem.
1st bug is that Album Artist is not transfered to the MP3's when syncing.
2nd bug is that when I switch back to the main library,
and then select sync, it wants to start back at the beginning, converting APE's -> MP3's.
If I restart MC then it realises straight away that the device is synced,
and says device is already synced.