My dog is terrible for barking when outside in garden.  If I shout no he stops looks at me then barks again running away when I tell him to come in.  I saw an anti-bark collar which sends out a noise do they work?

Used the spray one and they do work, don't know about the ones that make a noise, in fact, I don't know about the ones that make a noise, full stop.

He's barking for a reason - maybe it's attention seeking, maybe it's to ward of what he sees as a threat......  Either way, stopping him barking without teaching him what you want him to do instead may well lead to some other unwanted behaviour coming out. If he's barking to defend his territory when he stops and looks at you offer him something that will make him want to come to you (he can't concentrate on what he's barking at and not fall over his feet) when he's quiet make a fuss of him and let him go back out (he won't come to you if he thinks you're going to take him in because that's the end of all his fun).  This will break up the cycle.

Some just bark because they can and like doing it, I have 4 of them.

One of my friends has used a spray collar with great success on all 3 of her dogs, but the same collar on my terrier was a disaster.  The first time he got sprayed for barking he became very anxious.  I'm not sure if it was the spray itself, its smell, or the noise it made when spraying....but he really hated it.  He retreated to his bed shaking and growling, which meant he got sprayed even more.  It took him a good few hours to recover after I'd taken it off him.  I've made him a promise to never use one again! 
I've reverting back to just persisting with praising him when he quiet and he's much improved.

And some dogs don't even notice the spray (odourless or scented).  Try to borrow or test out a collar before you buy one as they can be fairly expensive.

As some of the others have said, if you know why your dog is barking you may be able to find a solution that doesn't involve a collar. In some cases collars only hide the problem rather than solve it.

i personally would not use them i thought the reply from digger good advise  there are all sorts of traing  methods you can choose from  and i know its not easy i would prefer them .you say if you shout he stops looks and runs away when you tell him to come in .all my dogs understand the word "come" because i trained them are not saints though far from it i have had dogs for 11 years now .