I have been having casual sex with this guy for 3 years now, Even i though i am currently living with someone and have 2 young children.

Whilst I do not really recommend this situation.  At least you are keeping your partner and children together, so there is some benefit to it.

The other guy does know my situation but we never talk about it. Its like i have 2 seperate lives, i care for the partner I am living with but im constantly thinking of the other man in my life, we talk on msn, some weeks more than others as he is very busy working all the time, he moved abroad 1 year ago but when he comes to the uk we meet up and spend as much time together as i can, and we have such a great time and then im in tears when its time to go.

It sounds like a brilliant friendship with sex to add to it.  And that's probably what he considers it to be, sex buddies.  You have your partner and family, and have a bit of fun with him, he has a bit of fun with you without having to commit to a family or full scale relationship.

But the last couple of times we have met up, once we slot back into our other lives he tends to go really quiet for a couple of weeks, i have usually sent an email saying how much i miss him and wishes he were still here etc etc and he will reply but this time i havent as i dont want to come across as an emotional wreck. I cant understand why he does this.

Because as I say, he is not committed to having a relationship with you, he wants the sex buddies bit, without any committment to having to do all those relationship things like staying in touch every moment of the day, having kids, it's just a life of fun without committment for him, so why should he change that.

He knows how i feel but I dont really know to be honest how he feels about me as he isnt very open like that. I just dont know what to do anymore is he like this because he wants me to leave him alone, but then i question it when he starts talking to me again. please can you help.

He obviously doesn't want to leave you because it sounds like he has a lot of fun with you when you do meet up.  Really he doesn't sound that interested in any relationship aspect, but as a friend I think he should still be staying in touch.  What he is doing is you are get emotional and maybe a bit attached to him.  He sees you getting attached and thinks you maybe want a more serious relationship and he doesn't want that, so he is going cold on you when you get like that.  Without words he is indicating to you, that he enjoys meeting up and having fun and sex, but he doesn't want any other commitment and he doesn't want you to get attached to him because he doesn't really want to get attached to you.  He wants to keep it as friends or sex buddies.