NYE I went out and found myself at one point standing by the bar on my own just having a breather and saw a guy approach a girl who to be honest looked kind of tipsy.

So I stood there just watching his approach to see if I could learn a few things or pick up on anything. He looked like he was doing well but then it looked like she wasnt interested and turned and looked at me. So I gave her a smile and next minute she is dancing her way over to me slightly and asks me to "save her" from this guy.

So we get talking and lots of kino and start kissing. Now at this point I think she is up for coming back with me. That night Im staying round my cousins flat who he shares with his flat mate. Anyway I try and lead and tell her she is coming back with me, she thinks for a bit and then starts looking for her friend (to either tell she is leaving or to run from me ). At this point I think its wise to get the keys off my cousin who turns around and says no to my request! I then turn around and find the girl talking to some other guy.

Now apart from the blatantly lack of preparation with regards to the flat keys, what is the best method to get the girl back without 1. losing her and 2. best way to ask her to come back?

i think she may have not been ready yet f-close. when talking to her you should have set up some imaginary dates in the future soo she doesn't see this as a one night stand about to happen but sees you as a possible mate. now when you see your girl talking to another guy like that you should learn how to amog! i read something about this but im still trying to improve on other parts on my game before going into that. The whole purpose of it is to go into the conversation and be the alpha male this should work. id look for more info on that . In the book the Game by neil strauss he has a chapter that covers it with some examples but goo luck.