But I have some theories about a certain man's body language. Some seem straight forward to interpret, some seem more mixed signals & is really doing my head in what they might mean! I was wondering if you might have some ideas what his body language may suggest he's feeling, in case I've understood the situation totally wrong???

While he's driving & I'm sat in back passenger seat, he smiles & stares me right in the eye for ages, thro rear view mirror.

If he was just staring I would say that was normal.  People like to see the other person when they are speaking to them and a rear view mirror allows them to do that.  In this case he is smiling and looking at you in the eye, that is pretty much close enough to doing the real thing, so I think he likes the look of you.

Seems distracted or stops talking/ignores other people he's talking to at the time when I pass, to stare or smile at me.

A man can only concentrate on one thing at a time.  When you appear he wants to concentrate on making a good impression to you, you distratct him.

Spots/picks me out of a crowd & eyes follow where I go.

He wants to check out what you are doing, make sure you are not getting up to any mischief with anyone else.

Lots of gaze holding- warm/admiring smiles & also get v intense stares also.

He likes the look of you and can't keep his eyes off you.  He's falling in love with how you look and how you make him feel.

Sometimes when talking, he laughs loudly at something I say & also often has his eyebrows raised like he's very amused.

Laughing loudly is his way of getting himself noticed, demonstrating to you he enjoys your company and maybe also demonstrating a bit of nervouseness because he fancied you, laughing loudly also demonstrates his strong masculine voice.   The eyebrow thing, is basically saying I like what I see, so I need to open my eyes wider to see even more.

A few times he's stepped into my space, has a twinkle in his eye & looks me right in the eyes while blushing.

Blushing well that's his bodies reaction to seeing an attractive woman,  the blood is running from his stomach and other parts of his body to the essential attractions parts of the body, like the lips, face, eyes and probably down below as well, if you know what I mean.   That's probably why people get that butterfly feeling in their stomach when they see someone that's attractive, because their blood is rushing from their stomach to other essential parts of the body.  The twinkle in his eyes, well that's a sign of attraction to, I know what you mean almost like his eyes are shining like a star and beaming a ray of light.

Then stops smiling/looks as tho he's done something he shouldn't & quickly steps a bit further away again.

This is his brain becoming conscious of what he is doing.  The blushing and everything like that is a natural reaction of his body.  Once the blood rush from his stomach occurs and the blood rushes to the other parts of his body, he becomes conscious and nervous.

When I give enthusiastic hello/g-morning, calling him pal-nickname, he blushes.

When women use a seductive voice to say hello (a long drawn out helllllllo), it can be very powerful to a man, it's enough to make him smile. 

He always returns hello, copies similar manner what I say,

So he's mirroring you! 

but when saying my name his voice goes soft & suggestive/deep. Noticed generally his voice goes more affectionate/puts more feeling in when addressing me or saying my first name, compared to speaking to others.

This is exactly what you are doing by giving him an enthusiastic hello, he is mirroring you and trying to make himself sound positive and attractive.

Sometimes if I don't look at him or speak when passing (due to feeling bashful/n confident), I sneak a look & notice he's scowling to himself! Several hours after he seems unusually moody/angry.

He is mirroring you again, you are giving him negative feelings, so he returns with negative feelings.  He's letting you know he isn't happy with the way you are acting, sort of teaching you what he doesn't like you to do and getting his revenge by doing something similar back.

He drops out digs that I know he's aiming at me & nobody else. He gives attention, praises & speaks to others, but singles me out -totally ignores or doesn't compliment me. If I don't bite to his tantrum, he paces about room & eventually goes quiet. He tries to speak to me hours later but when I refuse to accept his help, he goes very quiet & gets a serious look on his face.

Disappointed and unsure that you fancy him, he is frustrated that you have not only once ignored him but you continue to ignore him.  It's making him angry.

Looks back at me as he's going through doors, to see if I'm watching him leave the room. If he catches me looking generally, he stares & smiles at me knowingly, until he gets me to smile back/blush or look away first.

Keeps on digging away until you give him something to be positive about again.

Once he was talking to a friend, he stopped talking & smiled at me, then he & his mate exchanged smiles & kept looking/smiling at me.

His mate knows he fancies you as well.

When talking/smiling at me, he smooths shirt/chest or tie, fiddles with an earlobe (usually does this alot when laughing & blushing at me), stands with his hands on his hips, stands tall, chest out shoulders back, smooths or strokes outer thigh & smooths hair.

Demonstrates many things, he fancies you a lot but is also very nervous about the fact that he fancies you.  He can't control himself and stop himself giving off classic signs of being attracted to you. 

Normally he does not fidget, displays(alpha male mannerisms) very much in command & confident. He has never touched me. But when I touched his arm he didn't move away. He has as I stated earlier a few times invaded my space, but quickly backed away.

He's nervous that he might be moving too quickly for you.  He doesn't want to ruin things with you, so instead backs off, as if to say, I'm not trying to invade your space, but I will see if you move towards me and invade my space, then I'll know that you don't mind.

When he see's me, makes silly excuse to talk to me & seems reluctant to break off conversation.

That's because he wants to talk to you all day and all night, knowing blokes preferably all night.

He's done the leaning into me thing, stared intensely at me & also turned & done a double take on me, I think to see if I looked at him when passing.

Yes, you are right there.

When walking with another man, he wouldn't look at me, but winced/appeared a bit awkward.

Probably hoped you didn't check out the other man, thought the other man might spoil his chances with you.

Caught him staring at my figure(breasts), he seemed embarrassed/blushed & he quickly looked away.

Checking out your body to see if he can imagine playing with your breasts, see if they meet his standards or match his preferences.

Do you think a man would behave/flirt in this way towards a woman, if he wasn't available or interested?

He certainly fancies you, if a guy displayed all these signs and you didn't realise he fancied you, you must be daft.  As for the exact question you asked.  Whether he's available or not, if he fancied you, these are all signals you would display if you fancied someone.  I think he's more likely to be available to display these signals, because I think guys who are taken are more likely to try and resist women and resist giving off signs, or go hot and cold (cold when they remember they have a girlfriend). 

I would be curious to know in your opinion Sam, what this particular man may or may not think of me.

He fancies you and fancies you a lot.  You make him nervous and that's why I think he fancies you a lot. 

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