Recently my best mate has been dating a girl. She has been coming between the two of us. I used to be able to talk to him whenever I wanted and hug him and things but now she will find any way to stop me getting near him. He doesn't see what she's doing but everyone else around us does. He says she's not doing anything and has promised to spend more time with me but he isn't. I really need him at the moment but it's just impossible to talk to him. I really don't know what to do because he just won't listen. Can anybody help me?

Let him find out on his own if you try to tell him what you see he will resent you and she could say things like oh he is trying to break us up how could you ever like him etc... I would say even though it might hurt ignore them go find others to hang with show him what he is missing and he will start to see the real side of her.. she will screw up somewhere they always do... so move on but not to far and let him see her real side and in the prosess you could make some new interesting friends.

Well your friends girlfriend is way out of line.
I can understand where you are coming from and you have every right to be upset, maybe you should pull your friend out of his gang or maybe call him or even send him a text to let him know what is going on and how it is making you feel, then ask him if he can arrange a day for just the two of you.
Talk to him help him to understand but the best way to do it is to remind him of your good friendship before she came along.