Eyes,
You mention Heathrow as an example of fees, but, seem to almost deliberately ignore my STN example, which, is surely more relevant given its now a MAG airport and therefore a better example of how they may wish to operate?
Since MAG has taken on STN, Ryanair have increased flights and passenger numbers by a large degree, even to the point STN may once again overtake MAN as 3rd busiest airport.
They have also gained at least 2 new cargo operators, new airlines and more flights. Does that give the image of increased fees which may put off carriers?
I'm not entirely au fait with exactly how MAN will pay for the terminal re-development, but, given you seem to be focusing on fees in particular, these are the best answers that can be given.