There are restrictions for late running aircraft landing overnight but in reality as this occurs infrequently easyjet would never hit that quota imposed by the council.
Certainly in the recent past late running aircraft have diverted away from Southend rather than land in the early hours so while
chesna152 mentions that there is no official curfew for whatever reason there might as well be.
The airport does levy a night surcharge rate and extra fees for fire cover but whether that is enough to put airlines like easyjet off from landing in the early hours I haven't a clue but it is a fact that while some aircraft have landed just past midnight those arriving at say around 2am go somewhere else.
Looking at the departure board it looks like another frustrating day.
Not just for Southend but other airports as well but sometimes it seems that Southend feels the pain more with its frustrated supporters here.