I moved to Oxford just 4 months ago, so I don't know if Chiang Mai has declined, but it's definitely not good now -- to put it mildly.
Right next door, AT THAI is easily the best Thai restaurant in Oxford, and maybe even the best restaurant in Oxford, period. Even by Los Angeles standards, their versions of various Thai standards measure up. All of the noodle dishes are excellent. I would especially recommend the duck basil. I eat at At Thai at least once a week. Very reasonable $, er I mean £.