Our garden backs onto a lovely field and so we have the opportunity to just let our dog run free when we take him out. We take him out twice a day and although most days he's fine occasionally because Rusty thinks of this area as his own he will challenge other dogs (but only dogs slightly bigger than himself). This morning he ran after & went for another dog who was completely minding his own business and it has really worried me. He didn't hurt the other dog but he did jump on him and his teeth were bared. When my partner shouted at him he backed off straight away.
He is a brilliant dog and 99.9% of the time he's well behaved and gentle. We are guilty of spoiling Rusty and we realised that he was getting too big for his boots a while back and so we have stopped letting him on sofas, beds, etc but he still lets himself down by this behaviour out on the field. He can be very protective of his toys, his food and basket and of us and will growl and bare his teeth id he feels either is under threat.
Is there anything we can try to stop his aggressive side?

Try reading a book called the practicle dog listener by Jan Fennell it gives advice on all aspects of dog behaviour that is dominance based , and to be fare thats all it is , he still thinks he is boss , some dogs dont take this very well as it stresses them but if he feels that you dont make good enough leaders he will opt to be alfa . Doing the whole sofa and bed thing is a good start but there are lots more to it than just that , such as eating before him even if its only a gesture of eating a biscut befor you put his food down . Maybe for the time being you should have him on an extendable lead untill you can trust him again ,

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absoloutely, there are lots of things you can do around the house and on his walks that will remind him which rung of the ladder he is on. Putting him on the lead and walking out into the field, taking the lead off and "sit and stay" until you say he can go is a great way to start off the lead training...
Dog control is just that, you have to be aware that something may happen at any time, think of it like gears in a car..if he begins to run toward something/someone you have to get him under control again before he gets out of second gear or it's too late...lol

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"No one appreciates the very special genius of your conversation as the dog does"