I suspect there are more than sufficient slots for a significant Jet2 operation. FR used to base 42(?) aircraft, now it's 34/35?. EZY as we know have lost 4 or 5 aircraft. Yes FR operate more from overseas bases now but even some of those slots were broadly used by Air Berlin, Germanwings, Skyeurope and others when STN was at its previous peak.
FR's night arrivals are also concentrated between 11-midnight. From 9 until 11 there is a lot of capacity, also after midnight. Looking at MAN and LBA that would seem to suit Jet2 who have just one arrival at each between 11-12 and actually quite a few before 9pm and after 1am.