Can anyone tell me what a certificate of merit is and where it comes in the placings at working tests etc.

Placings are awarded for 1st to 4th place in gundog working tests.  COMs may also be awarded after 1-4 at the discretion of the judge.  It is worth mentioning that judges may withhold placings if they do not consider the standard sufficient to win a place.  This can happen in working tests, however, due to the number of entries it is not common.  It does happen more frequently in field trials.