I am not sure if a guy I work with is a really nice guy and fancies me or just a really nice guy being friendly-

we sit opposite each other and chat a lot- he asks most days what did you do last night, at the weekend- what are you doing at the weekend etc. he has asked about my family, places I want to go etc. He has gotten Sex into the conversation a couple of times- as have I

It sounds as though you have got some mutual conversations going.  I would say, that if he doesn't fancy you, it is only a matter of time before you build up so much rapport with each other, that he would have to at least start to fancy you.  In general guys don't engage in idle conversation, unless they know they are going to get something out of it.  The only time guys get into idle conversation is if they are stuck in the staff room and the silence is just too much for them to bear.  So they make conversation to break the silence.  But to actually have a conversation with someone for a long period of time and a variety of different subjects it kind of builds up a rapport.

It's very difficulty getting a good conversation out of a bloke.  Even if he didn't fancy you at first, if you have enough meaningful conversations out of him, the rapport can start to build up, and he can start to think, this lady really is something.  Men don't find it easy making conversations, nor do most men find it interesting to make a conversation.  So if women manage to force a bloke into a conversation and actually get them to enjoy a conversation, it makes a man take serious notice of the woman. 

Most men get used to trying to make a conversation with a woman, only to be either told, get lost loser or to find that their conversation doesn't interest the woman at all or find that they are interrupted by other women who constantly butt in on conversations.

We have been out on site for most of the past week- just he and I in a car at various seaside locations for work- In the car- chat is easy, i make him laugh and he makes me laugh

Laughing is good, and spending so much time with each other is good, especially seem as though spending time with each other seems to be fun for both of you.

If I mention a bloke friend he is quick to ask who is he etc

Yes, I think you can safely say he is interested in you, pure jealously or making sure you are not seeing anyone, which increases his chances.

I've also caught him looking down my top a couple of times- all positive or so it seems-

Yes, it seems he wants a closer look at what's down your top.  As long as he gives you some eye contact as well (because if he doesn't give you some good eye contact, then he might only be interested in what you have in your top).

I texted him a couple of times after our days out- just funny observations but he didnt reply, or even acknowledge that I sent them- he mentioned the last one but looked surprised that I'd texted him- I took that as a very bad sign and that he was just being friendly before

No, that is not really a good sign.  But women need to be careful that they do not get too close, too soon.  Like making possessive comments or making emotional or feelings statements.

we work together- same grade, and I organise the work night outs so I've organised a few- the last one I was sick and he noticed I had gone before other people and asked about me and told me he'd asked where I was. At the first night out he stood beside me for most of the night and a mutual friend told him there was a spare seat beside him but he said he was ok where he was.

It seems that he is very attached to you and enjoys being with you and is comfortable with you.  Whenever I go to conferences I like to stick with the people I get on well with most.  He obviously wanted to stick with you, either because he got on with you better or because he didn't want you to be sat on your own.

At another I looked up and he was looking at me and I know what he is talking about most of the time without him saying much. another time we were chatting and neither of us had noticed that a colleague had moved to sit beside us and had been watching us for 5 minutes before I noticed. He gets a bit embarassed sometimes too and remembers what I have said in previous conversations etc and asks me lots of questions

It seems you are almost like a married couple that know each others thoughts and preferences, which is a good sign.

he has however started more about himself- pointed out where his relations live etc when we were out onsite and was telling me about when he was at school..

Men, find it easier to talk about themselves, but very difficult to ask the right questions to women, and most of all find it very difficult to listen to women.  Particularly as women tend to talk about 8 different topics in one paragraph (a man can't multi-task, so he can only focus on one topic of conversation at a time). 

I had thought something would have happened by now so dont know whether he thinks of me as more than a work colleague-

It's true if he is shy or a nice guy (that doesn't use women for sex) and values your friendship, that he probably would find it difficult to make a move.  But men also value their jobs a lot aswell.  And having relationships with work colleagues is classed as very risky.  Do you risk having a relationship with a work colleague only to find that you could lose your job or make it very uncomfortable to do your job (if you split up or have an argument).

You give me a mixture of points for and against him fancying you.  I would say that you need further evidence before you know whether he fancies you.  You need to give him some tactful indications, that it is ok for him to get closer to you.  If he touches you or moves close to you, make sure you don't show any signs of moving away or looking uncomfortable.  Try to get touching him, but only infrequently and try to make it look accidental.  You should instinctively know when you have the chance to touch him.  He could be joking about something, when you are sat next to him in the car, just give him a light tap or stroke of the arm.  Particularly if he is using teasing jokes to wind you up, the light tap on his arm looks more like a natural instinct that women have.