The TVDB ID is populated in the tags along with titles and the Access Rating. There is no Description though, but I can't get onto the TVDB web site to check as it's "undergoing maintenance".
I can't see anything wrong with the permissions. The directory has exactly the same permissions as all the others and, under the same account that MC runs under, I can create new files in that folder and rename etc.
However, I did think there was something not quite right withthe TVDB lookup although I accepted it at the time becuase I thought that was the best it could do. The boxed set is actually season 1 of the Banana splits (yes, I know!). When I first imported them, the lookup stored the title of the particular episode of the Three Musketeers that was included in that show. So I've now tried to Get Movie & TV Info separately on each file and there are two entries returned - one is the Three Musketeers episode in position 1, and the other is the proper one of the episode of Banana Splits. So I've selected the second one for six of the files so far and re-run auto import, and it's only now re-running Get Movie & TV info for 30 of the files. So it looks like that is the problem.
But..
a. MC's lookup in the first place isn't getting the correct TVDB entry (even the file is named correctly according to convention as "The Banana Splits.S01E01.Title.mkv".
b. MC somehow knows that it didn't get it correct first time around and tries to re-do it each time, but still gets hold of the wrong info.