@Rob **********
ahhh Chiang Mai is FAMOUS for jerking you around to give you an 'incentive' to use one of the agencies who charge 500-1000baht to get the certificate the same day
You are not alone in your plight. Go back to the immigration office and ask them about it and see what they say, THEN go across the street to the agency and pay them the 500 baht I think it is so they can get a certificate of residence for you in a day.