Viv, your post has reassured me that my Monty is not the only one with this problem!
He is almost 5, and started the defensive/aggression thing with male dogs a few months ago. We used to assume it was purely defensive behaviour, but it's got worse recently...he is more the aggressor and it's getting embarrassing!
At home, and with people, he is a sweet dog who is not at all aggressive. I walk him on a lead as he can get excitable and easily distracted off it, and was once hit by a car.
He went to dog training when younger, with little 'success', e.g. he thought it was Christmas and his birthday when they threw toys at him...(he was supposed to ignore them!), so I don't know if training re aggression would work...however, as his coat already makes him resemble a womble, I'm reluctant to risk the side effects of castration!