FR probably had a contract at BLK. but that did not stop them going.
3-4 routes that FR can pull at the drop of a hat is not the same as an investment in a base by easyjet by any stretch. The assumption that airlines pay the published fee's, particularly at the smaller airports, is naive.
At the moment that is pure speculation as there has been evidence here that not all services had good loads.
Except I think its OK to accept that easyjet
wouldn't put a fourth aircraft into SEN next year if it was falling below expectations.
Just my 2p worth.