I'm travelling Ryanair with no bag drop at the end of September from Stansted airport. My train arrives the night before. Is it possible to go through security when I arrive and sleep overnight when I'm there? I've read that Stansted isn't sleeper friendly but I plan on just sitting at a bench overnight, is this allowed?
Yes. I've done it several times, the most recent being July 2019.
It depends when you arrive to if you can go through security though. The airport is 'shut' for a few hours in the middle of the night.
Basically, you can arrive anytime and stay in the arrivals/departures area before going through security, there's loads of floor space (not a lot of seating) and some people even use sleeping bags and eye masks.
Last time I was there security was closed between 12-2am, so you won't be able to go through until after that time. After you go through there's a lot more seating and again, quite a lot of sleeping people. Security make their rounds and sometimes wake people up to check passes etc.
A great source I use for things like this is the sleeping in airports site, heres the one for stansted.