I am in a dilemma... I really fancy this guy that works at the same company as i do.. I am sure that he fancies me also as he always looks at me when ever he passes & always has sneaky glances when he can, (so i have been told by colleagues).

If you and your colleagues have noticed, I would say it is fairly conclusive.  One person noticing is not really conclusive but if others have noticed as well, then that makes it more definite.

This has been going on for months now, I done a silly thing a while ago, We all went out, the company, I and got very drunk...sat down & told him I fancied him, he was very shocked & asked me why would someone like me fancy him, as noone fancies him??!!!...I am not sure what he meant by that,

A guy lacking self confidence, when it comes to attracting women. Usually because he has had a lot of bad experiences with women.  Women tend to be very cruel, when it comes to rejection.  The words, "get lost loser", "do I look desperate", often come from a woman's mouth, towards a guy.

only he told me that I was too good for him, in not so many words, he did tell me in a drunken state though that he really fancied me also!

When people are drunk, you can never take anything they say seriously.  When people are seriously drunk they see any male or female as attractive and say silly things.

We later left the club together...(a few things went on but we never had sex...too drunk to remember, but I am sure that it never happened).

Cheap, one night, drunken flings really are not that good for people, so glad you both resisted.

I am a married woman but in a very unhappy situation at present, (although we are trying to resolve issues we have),

You can be unhappy one moment and then happy again soon after.  Though it is quite difficult to stop the rot, when a marriage is on the rocks.

I feel really silly & guilty now, that this has happened with this guy,

I don't understand why you feel silly or guilty.  You didn't have sex and as far as the silly goes, both of you wanted to do whatever you did that night, what's silly about that?

I have asked to meet him out of work & he has told me that he thinks it would be a bad idea, (I am glad that someone is being sensible), as I am married etc.

Yes, women tend to be very impulsive sometimes, surprisingly most men tend to be more sensible in this situation.  Most men tend to avoid married women, there is too much baggage.  Too many hurdles, like one big hurdle, your husband, if he gets into a fight with him.

(I know that he has a girlfriend)

Now you know why he is even more sensible.  There are four people involved here, you, your husband, him and his girlfriend.  This makes it too complicated for a man.  Most men prefer to keep it simple (when it gets any more complicated men tend to cock it up).  He could dump his girlfriend for you, only to find that your relationship with him doesn't last.  Now what is he left with.... He's lost two girlfriends.  He will know his current girlfriend better than you, so you can bet that he will take the safe option of his current girlfriend.  The only chance of this changing is, if he is unhappy with his girlfriend.  And you have not indicated that this is the case, so I doubt he is unhappy with his girlfriend.

he said that if We were both in a different situation, then he would jump at the chance, but he doesn't want to wreck anything!

Yes, this just emphasizes what I have explained above.  This guy is obviously an honest genuine kind of man, because he is giving you a straight answer.

I feel like there is so much frustration between us... he is still looking at me even more so now... he is speaking to me which is good, but he can't walk past the office I am in without looking straight in & looking at me.

This emphasizes the fact that he really does fancy you.  And it will be a bit frustrating for him too.  If a man finds out that a woman fancies him and he doesn't want her to fancy him, then he will avoid any contact at all.  The fact that he still talks to you and looks at you, demonstrates he is genuinely fond of you.  You can keep your eyes off, things you find attractive.

If I look up then he always has a big smile on his face & he waves!! I am sooo confused!!

I think you are both confused, and anybody would be in this situation.  Your instincts are saying that you fancy each other, but your morals are telling each other it is the wrong thing to do.  And it's best to listen to your morals in this situation.  There are too many obstacles to overcome before anything can work.  He will see a great barrier with the fact that you are married.  If you had been seperated for a while that may make it more easier for him.  But you are definitely not at that stage.  And you can't rely on him to stick around for this stage.  Because you could dump your husband only to find that your relationship with this guy doesn't work out or doesn't even happen.  And again you will be without a husband and without a boyfriend and totally on your own.

Please just tell me to grow up & get on with my marriage... I need someone to tell me how it is!!

Everybody goes through rocky patches in their marriage.  You shouldn't be too impulsive about leaving the marriage, unless it so bad that it is unbearable.  You have known your husband for far longer than you have known this other guy.  So there is no point in going on a whim, and finishing your marriage, for a guy that you are only just getting to know. 

What you should be doing at the moment is concentrating on fixing your marriage.  You have got time to decide whether your marriage can be fixed or whether you need to get out of it.  This requires a fair amount of time, you can't just decide this over a matter of days, it must be decided over weeks and often months.  It maybe that after those weeks or months, you decide the marriage is unbearable, that is fair enough, then you should find an exit out of the marriage.

There is no problem being friends with this other guy, or chatting to him, just don't do anything silly and impulsive, which will shatter any chance of ressurrecting your marriage.  You can keep this other guy in your pocket should your marriage become unbearable.  Then if you make the decision to finish your marriage, I'm sure this other guy would find it much easier to go with you.

So grow up and concentrate on your marriage for now, if this doesn't work out then you can start looking at your other options.  Keep it simple.