Forgot to mention it earlier but there's a thing over here called the halti which a lot of people swear by. I personally know lots of people who have used them successfully on a variety of dogs including mastiffs and rotts. Basically it's a collar with an extension which goes loosely around the muzzle allowing the lead to be attached to the muzzle end - when the dog tries to walk off in front/pull, his head is turned this stops him from pulling. Most people use them for a while until the dog is walking correctly then transfere to a normal lead. here's a link
Halti Headcollar to Stop Dogs Pulling and to Calm Boisterous Behaviour
which explains it in more detail. If the choke chain hadn't worked for me I would definetly be buying one of these.