Nick... I seem to recall you answering a question on here about office romances before and that they're not a complete stranger to you.
That being the case, be honest, do you get a sense that she likes you? You mush have an inkling?
You're looking in to it too much, I don't always respond straight away to my emails but it doesn't mean I don't like someone, just that I'm busy (or occasionally grumpy). Women are constantly bombarded with advice not to look too keen too so it may be that she's worried about coming across that way.
I know it can be a bit difficult if you work together but if she's as nice as you are she's not going to make you feel bad for being interested. You wouldn't do that to someone so why would she? Even if she doesn't like you that way she'll probably be very flattered, she's hardly likely to rip your heart out, kick around the pitch, head it in to an open goal and then hand it back to you covered in mud now is she?
And you can't let fear of rejection hold you back otherwise you'll never have a date again!