New Horse owner - could use some tips.

I have ridden horses for the last 9 years and worked part-time in a riding school for the last 5 years.
I always wanted to rescue a horse and train it up because training horses was always something I always loved doing and I rescue a lot of animals. So I bought or rather rescued a lovely natured 6 year old 13.2hh gelding cob type mongrel. He was very scrawny and his feet need trimmed and some TLC but he is looking better already.
In his previous home he lived in the back of a lorry (made into a sorta portable stable) in a small field, he has no papers or passport and the previous owner doesn't really know anything about him or horses or the ponys history. The previous owner bought him at a fair and he was ridden August + September 2010? And then left in the field, he is the best natured horse for one that has not been handled a lot but is still a bit green.

Ok,
So he opens his mouth a lot when I'm riding like he is uncomfortable with the bit and lifts his leg up to his mouth often when I'm riding and flicks his head, I think it might be his teeth (I am having them checked ASAP) but a friend suggested I try a flash, a hackamore bridle or change the bit any opinion here?
I've tried adjusting the height of the bit and checked the bridle, he may just not be used to the bit yet but I have stopped riding him till I get this sorted out just in case its something more.
Oh and also when he's in his stable he makes the noise a horse makes when its wind sucking but he's not wind sucking on anything, it's like he is wind sucking on his teeth or something. He did it on the way in from the field yesterday could this be relevant?

I know I have to get him microchipped (or checked for one), shod (and feet checked etc.), vaccinated, get insurance and apply for a passport and I have already wormed him (he had red worm).
Anything else I need to do?
P.s I am in the UK

Any helpful comments and tips appreciated :)

I wouldn't go for flash personally. I'd say start with teeth floating/check and hold on till its done. If it won't help - look into different bit. What kind of bit are you using now? And are you sure the size of the bit is correct?

I agree with kitten. Period.

Yeah I hate flashes. I would see if the teeth help. Also I use loose ring snaffles with my horses and they LOVE them. The D ring is more stiff and I find does not work as well.

I'm not big on tack knowledge but the bit looks an ok size to me but I will definitely get his teeth checked first and then maybe try a new bit. I'm not a fan of flashes either, I found the last horse I rode with one was worse behaved with it on than with it off.
I'm pretty sure its an Egg Butt Snaffle bit, it is the bit that came with him and that his previous owner had ridden him on last year. http://www.elysianfieldsranch.com/im...uttSnaffle.jpg

Anna, many horses like 3-links bits (like this one: Dover Saddlery | AlBaCon French Link Eggbutt Snaffle Bit . ) over the one you posted. With that being said you never know until you try.

Kaylee, the rings also depend on horse quite a bit. My paint likes loose rings, my qh hated loose rings so she goes in eggbutt. I use oval mouth on both of them (like this one: Dover Saddlery | Mikmar Cupreon Eggbutt Snaffle Bit w/ Ergom Lozenge . )