Southend Airport Web site say 23:00 cutoff.
"Whilst passenger flights may only be scheduled between 06:30 – 23:00 (a limited number between 23:00 – 23:30) delayed aircraft may still operate back to Southend during the night-time period.
This was considered as part of the planning application for the runway extension; Southend City Council, Rochford District Council and Essex County Council agreed that as London Southend Airport had always been operational during the night-time, and was already a diversion destination for other airports, and it was reasonable for delayed passenger flight to be able to return passenger back to Southend when there no other suitable alternative airports in the London area."
That must put it at serious disadvantage vs other airports in making maximum usage of and airplane during the day and thus the airlines bottom line.
Looking at tonight, Jet2 have 10 arrivals at MAN, 6 at LBA and 3 at STN after 23:00.