[vbat99]

Thanks. I've tried again to add an overview for a custom set, without success. I ended up adding it manually - it's seen in kodi but not in MC now. I added some files to a TMDB set that shouldn't have been in it - the overview seemed to be written to some .nfo's but not all.
If I know what the expected behaviour is I'm happy to do some testing.

So the issue is that we store custom sets in the config.xml file, and TMDb sets in moviesetcache.xml. As TMDb sets have a 'Set' Id, we can store all movies that are associated with the 'Set' and the overview if one is available.
Custom Sets, there is no set ID.

I'll have a think on how to implement adding Overview to Custom Sets, so that it will populate all movies associated with the customer set if overview is edited.
Then will have to figure out a way to add overview to any movie added to the custom set.

Interesting, but not impossible.