Broken retaining tabs on the JAN II are very common, but really only a problem when the strings are off the bass, otherwise string tension keeps the nut in place just fine. The main difference from JAN II to III is they got rid of the tabs, so there's nothing on there to break, but at the same time, nothing to hold the nut on when the strings are off as well. If you call up the US distributor, they might send you a new JAN III for cheap. You would have to pop off the old one with a knife blade, clean off the previous glue and superglue a new one in place. There's also the new brass(!) JAN III nuts that are around $140.
If it was my bass, I wouldn't pay any attention to it, a broken JAN II works and sounds the same as an intact JAN II or III.
There's lots of good info & good people to be had over at the Warwick forum too:
http://bit.ly/cUkZag
