I feel that this dog is way beyond your skills as an owner. You haven't mentioned the possibility of a trainer, and that doesn't bode well, either.
Your dog is a ticking time bomb and he will eventually attack another human, child, or animal, and quite possibly kill them. Do not, under any circumstances, ever take this dog to a dog park. Your yard is the only place he can be off-leash. These are not nice things to say, but you need to come to grips with the fact that right now your dog is a liability and you have no control over your animal. I would recommend going through your yard and making sure there is no place he can jump over, dig under, or chew through the fence.
i have tried to calm him down when he does this but he turns and snarls at me and carries on
Snarling at his owner is not a good sign at all and I would not be surprised if you get bitten by this dog the next time you do this. You didn't describe how you're trying to calm him down, but it's not effective, and quite possibly making the situation much worse. Only a professional can help you with this.
makes him stressed and pulls extremely on his lead.
Make sure that he cannot pull free of his lead, snap the metal parts, or get loose of his collar/harness/leash. You are using your sheer body weight and force to keep the dog under your control, and this is an accident waiting to happen. If either you or your wife walks him and you are not paying attention, not feeling well, or are ill/weak, it's very likely the dog will get lose and bloodshed will occur. You will want to talk to your dog trainer about prong collars.
Other than that he is extremely loving and affectionate with me and my partner and follows us round like a shadow, we love him so much
This is not enough reason to keep a dangerous animal that you cannot control. You have a lot of hard decisions to make. Surrender is hard. Training will be long and very hard. You need to be honest with yourselves and determine if you both are capable of getting on board the trainer's program and devote all of the time and energy to participate in its rehabilitation. Special emphasis on both you and your wife, if only one person participates, the non-participant undermines all the training.