I know what you mean. I'm in a similar position where I really like this girl...but I'm too afriad to tell her. Fear of rejection probably keeps me quiet, but so does being too shy.
What people have told me to do (which I refuse to do...

) is go out for the day with this girl. Maybe to Manchester, or a little cafe somewhere, and just tell her face to face, with no-one else around.
The only thing to do with good advice is pass it on. Try taking her out somewhere, maybe to the cinema, a little cafe somewhere, shopping down the local Shopping Centre. Both of you together, no-one else. Talk to her a bit, get to her know her, make her day enjoyable. A day together will really make a difference. And then, if you think you're ready, tell her that you like her. Maybe if you aren't ready ask her if she wants to do it again sometime. Then tell her the next time your together.
EDIT: I got close to telling this girl about a week or so ago...but I chickened out. She still really likes her old bf. They're back together even...4th time or something.
But anyway, I was contemplating asking the same question, just slightly different. You've beaten me to it! I'll have to wait a few months before being able to post it without being accused of copying or starting a thread that's already around.