It could well be the case Jet2.com are taking more than custom from just STN based services & airlines...Jet2.com are advertising extensively throughout the South & as STN is their only Southern base it perhaps stands to reason they are pulling from more than just the STN catchment...You may well need to look @ LTN which you bracketed & with not too much hope for further sun flight growth @ SEN I would expect....time will tell...