Well that didn't help...
Downgraded to 5.1.12 and the problem got worse. Instead of adding a random second name to PMs it now adds three or four.
Tried flushing the caches, rebuilding everything and repairing the tables and it didn't make a bit of difference.
Assuming UBB answers my support request to clear up the confusion over my license number I'll let them take a look at it. At the moment this is the only installation on any server with a problem and it's turning into pure FUBAR.
