best of luck! There's rooftop access from the car park, you can climb around the fence they have put there but be warned there's a camera right in the path and the second you get onto the roof, quadrant security will waddle over to escort you off the property
Recently rembered about this place so decided to give it a try, ended up getting caught by security guards who (to be fair) were quick to respond to our presence. Access is easy from the car park, roof doors may be viable to enter from but didn't have time to check any, advise if your going to try go in the dark. Cameras are also there so watch out for those.
i managed to get up from the same place as you only a few months ago.
i tried all the doors and im pretty sure they had been barred from the inside.
either way as i was coming down security caught me but let us off with a bolloking.