Thank you everyone, some very good advice!
I did have a rescue dog before some years ago and unfortunately it didn't work out and we couldn't keep him. It was a couple of years ago, my eldest was only 3 and she was on a buggy board with the 2 boys in a double buggy. I did explain that I needed a dog that was used to being walked and controllable on a lead but this dog, lovely as he was, was a complete nightmare. He was huge and over excited - every time a dog came near he would do somersaults and it wasn't safe to take him out and not fair to leave him in the house so I had to hand him back. I'm not saying that all rescue dogs would be the same, but I would rather have a dog as a puppy and be able to train him myself.
Labradors are lovely dogs, I grew up with one and she was the most gentle animal ever. I did consider a collie as well, but thought they would need a larger garden to burn off a bit of energy in between walks.
Its a huge decision and I am going to consider it carefully - I'm certainly not going to get a dog for the sake of it, if I can't find one suitable then I suppose we will just have to make do with the fish tank!
thanks again. x