Nat Ex coaches
I thought that the best way to find out anything about the early/late coaches was to ask at the airport. I am now back home having done just that.
I only spoke with the lady on the enquiries desk but she did seem knowledgeable on the subject. It obviously varies from day to day but she seemed to think the coaches are doing quite well as they are convenient and the fare is only £10. When she has worked late shifts she has seen quite a few people heading for them. As far as she knows, they're "doing ok".
Sorry not to be able to be more specific.