lil hooligan said:
ye i would have deffo asked her out but she is always with a gang of mates, any thoughts on how to get around this?
If you can't catch her when she is on her own, grab her hand and say something like "Hey, come with me a sec." Say it confidently so that she knows you want her to come with you. If she asks why, don't tell her why. Just repeat yourself "come with me" or whatever. Then you can lead her away and poof! Now you can talk to her one on one.
Don't sound all sentimental with her though because that could create an awkwardness as she is probably not on the same wavelength as you at this point. From here tell her to put her number in your phone or write it down for you. You can suggest that she join you for ___ sometime if you want to (or you can wait and call for that. Know what you have in mind beforehand though.
Don't ask her when she's available or what she wants to do. Offer a time when you are free and something you want to do. If this is a problem for her, she can suggest a different time that is good for her. Don't count on her to call you. If she offers, tell her not to and that you will call her on ___ day. Then don't call her until that day.
After you're done with your little alone conversation (and don't keep her too long), let her go back to her friends. Don't go back with her, go off somewhere else. To your own friends, or class, or whatever you've got to do next. You don't need to follow her back. If you go back with her, her friends will probably ask what that was all about and you don't need to be there for that.
Any other questions?
edit: When talking to her, just pretend like she's an old friend. It's not a big deal when you're talking with old friends and it shouldn't come off as a big deal that you're talking with her. If you're nervous, she'll be nervous. If you are calm and comfortable, so will she be.