Maybe they forgot to ream the seat tube after brazing? Possible. I agree wait to hear what they have to say. If it just does need reaming some bike shops have a seat reamer or a local frame builder but get the bill sent to mercian, rather than post the frame back up to them. Try and get hold of some vernia calipers to mesure the seat tube your self if you can't wait.
Maybe they forgot to ream the seat tube after brazing? Possible. I agree wait to hear what they have to say. If it just does need reaming some bike shops have a seat reamer or a local frame builder but get the bill sent to mercian, rather than post the frame back up to them. Try and get hold of some vernia calipers to mesure the seat tube your self if you can't wait.