There are security staff but the gateline is back to very often being left open in the late evening. Same with Euston.
Euston gatelines are always open at night now I've found ,but I use the 0134 or 0035 out of Euston back to MK a couple of times of week , and have always found the barriers closed at the MK end.
Very annoying with the ticket barriers , that they reject a day return because the return time is after midnight , so you have to show it to the security staff to be let out