Sorry, just getting back to this now. I forgot about it for a while but came back to my attention. I did check the case and it was wrong. I corrected this but it made no difference. As an example, I have created a library view to show symphonies that are neither by Nordic nor Russian composers - i.e. all other symphonies albums. The view is by composer (using Composersort) and then I can click on each composer and then see a view of all albums for that composer. This works perfectly. However there is a group called 'Unassigned' - i.e. no composer identified. I checked the tags and Composersort is populated. The albums in the group are all either ALAC, AIF or DSD. So the only logical conclusion is Media Centre is ignoring the Composersort tag for these file types.
To verify I converted one of the ALAC albums to FLAC and put it back, and sure enough now that album appears under the respective composer.