Thanks for the warning. I had a similar experience, down in Tooting, some years ago - very unsettling! I didn't have the door chain on & felt very vulnerable. Just recently I also had a young man at the door, but thankfully he just stormed off after a big sigh & throwing his hands up at my rejection.
I would definitely recommend calling the police - what they're doing, if you have already asked them to leave, is technically illegal! And if you're worried about reprisals, perhaps say to the police that you don't want to be identified (in public) as you're scared of the consequences, but could they just come along and be a deterrent & move the guys on?
On Streetlife (another local forum I'm a member of) this post with some printable signs might be useful to readers...
http://www.streetlife.com/conversation/ ... 9b9000016/ - you can read it without being logged in.