Turbo thanks for the info. According to Peugeot the ring is part of the bearing. What search title do I type on ebay to source the toothed ring?
It appears that the 307 doesn't have a toothed ring, here's a thread I found on another forum which has some useful info about your problem.
http://www.rac.co.uk/forum/showthread.php?3589-Peugeot-307-ABS-fault