After many months I've finally figured out why the group messages were not being received in followers inboxes. It seems that the 'Discussions' subtype of emails had not been selected as default.

I went into Settings/Technical/Email/Subtypes and ticked the default checkbox which was previously unticked. Then I went back to the message groups and tried to send another group message. Still, the message was being sent to 'nobody' despite there being several followers already added.

I then removed the followers and added them again, and this time it worked.

I don't know why the 'Discussions' subtype was not selected as default as I don't think I would have deselected it, but there you go!

Hope the helps someone. It was driving me crazy.