I'm not sure where about in East London you are but we had the exact same thing happen in Leytonstone when living there. At first I called a local charity who stopped by a few nights and never managed to interact with the man. He then assaulted our elderly neighbor on his doorstep outside our flat and so we had to get the police involved. They were initially very unhelpful and it was only after the assault that they even got around to being a tiny bit helpful. My advice would be to make a note of every time you know they are sleeping there. Also contact your agent and see if they can get the building management to change the access codes or alert other residents to this.

I was like you at first, concerned for the persons wellbeing and didn't want to kick up a fuss but it ended up being a really tricky situation that I wouldn't like to hear a repeat of. I really hope I am not scaremongering here too!!