S'ok, I just bought him a leather collar, and have put in an extra hole so it fits him snugly. I think I was panicking a bit earlier but I've checked that he can't get his head out of this one. His old collar must have been too loose cos there's no other way he could have slipped it really, and with it being a nylon one I couldn't put in an extra hole.
A Martingale collar is (I think) a leather version of the half check. They're good for dogs whose head is the same width as their neck (Greyhounds etc) because unless the dog pulls, either forward or back, they're loose but they tighten enough to keep the collar on if the dog if it pulls back.
Ddog who is a whippety lad, has a rope lead/show lead - he escaped from a normal collar, has eaten his latest collar while wearing it, and an eat harnesses while wearing them. The rope lead is the only one he hasn't managed to escape from.
A collar should fit snugly and you should be able to fit a couple of fingers underneath. If it is too loose, the dog can get caught in the collar and the collar can twist. If there is enough slack for the collar to twist it is too loose. If you are concerned, it may be worth checking with a professional.
My puppy didn't slip his collar, I dropped his lead recently and he ran straight into a main road in to the path of an oncoming car. Luckily the car stopped in time, whether it be because he heard my screams or saw my puppy, I don't know. But ever since then, my pup goes out on a harness and I hold the lead extra tight now.
Glad to hear your puppy stayed by your side, what a good pup you have!