I am having difficulty figuring out if a guy at work fancies me or not.

There have been two occassions he has used an excuse to come into my office and chat, and it seemed obvious at the time he was not in a rush to leave. Good sign, or so i thought. I asked him out for a friendly lunch date, which went well, we chatted about everything. It ended on a friendly note, and I said we should do it again sometime, and he agreed. There have been a few emails, flirty emails, indicated by me, he seems quite keen to send the emails back and forth. I asked him out socially for a drink with some of the other people in the office which he has turned down - twice with an excuse he has a prior commitment, but said he would love to come out sometime. So I left the ball in his court by saying, when he felt up to having a drink sometime, to let me know. Well, nothing has happened.

If he really fancied you enough, he would have asked you out by now, you have given him plenty of opportunity and made it so easy for him it is unbelievable.  I think maybe he is the type of guy to keep you dangling and when he fancies some sex, he will make a move.  It maybe that he is getting sex from somewhere else at the moment.  But nevertheless this guy obviously doesn't value you enough, so you should move on.

I have taken a step back as i feel at this stage i am doing the chasing, and even though he has showed interest, I am not entirely sure if he is just plain shy, or not interested in gett! ing to know me better.

Even if he is plain shy,  you have clearly spelt out to him that "he should let you know when he wants to go out for a drink".  That is beyond shyness and moves into "the not that interested territory" you offered yourself on a plate to him, you can't do anymore than that.

I feel like i have read the signs all wrong. Its a shame as I really do like him. How can i tell for sure if he fancies me or not?

Don't think you need to tell whether he fancies you or not.  You should move on, he has missed his chance if you have given him a few days to ask you out.  He either doesn't fancy you, or is going through extremely bad times or is just plain stupid or is keeping you dangling so he can use you for sex.