This guy at work looks at me loads, we speak a lot. I often leave it to him to approach me so he does some of the running here and that way I know he must have some interest in me, he asks about myself all the time and remembers what i say.

Yes, I think from that you can safely say he is interested in you in some way.

I sometimes approach him and ask him about himself too as I'm hoping he'll get the message I like him.

Yes,  I certainly think you should start showing some interest in him before he gets bored or thinks your not interested, or before he finds someone else.

But how, apart from being blunt and telling him can I let him know I like him in a subtle way and not scare him.

You don't need to openly ask him out.  You can make him fall into a trap.  Have plenty of conversation with him, eventually if you are thinking right, you will be able to trap him into asking you out.  It could be that you ask him what he is doing this weekend and you say you like what he is doing or fancy doing that as well, it is then up to him to bite and say well do you want to join me.  Or vice-versa you think of something he might want to do, and tell him your doing that at the weekend and if he bites you can say he can come if he wants.

He kept winking at me today and going red when I spoke about undressing!!!

Enough to make any man blush!

and when I said his car was nice and I'd love a spin in it.

Sounds a bit corny to me, but at least you are expressing to him, that you are interested.  I don't understand women's obsession with a man's car, as if all men with nice car's are nice people?

How can I get him alone and tell me what he is thinking?

You just need to carry on having relaxed conversations, eventually the time will come when one of you will make a move.  Pretend you are just close friends initially, and once you have got closer, one of you will make a move.

and let him know I want him?

Just be fun, never give him the brush off and never give him an indication you want him to go away (even as a joke).  If he gets the feeling you are having fun with him, then he will surely think you want him at least as a friend and possibly much more.

Don't tell me to just tell him because I never could.