Hmmm, if you have known her for ages and are more on a 'friendship' relationship then it will never work. She will only ever see you as a friend. You could probably have sex with her, but it wont mean anything.
It seems she knows she has a hold over you and can get you whenever she likes. A lot of girls dont go for easy/desperate lads and they can sense it on a person too. Why not just ask her what she is doing one weekend and if she says she isnt doing anything, just slip in the question' Do you fancy going bowling or cinema or summat'...