I've flown out of MAN maybe over 300 times.
Many times a great experience, but also had some howlers.
All I can say is don't bother complaining to MA. The Management are not interested in "moaning" passengers and will only respond to airlines that make complaints.
In the unlikely event the airport responds to a complaint they will usually blame somebody else instead of taking responsibility for the whole shooting match.
I can only speak from personal experience.
I'd suggest you complain to Flybe and ask them for an explanation & for them to
follow up with MA.