I'm a novice so I wouldn't be comfortable trying to catch another horse whilst mounted - especially this particular naughty monkey. Mind you, I wouldn't be comfortable trying to catch him on the ground either...
I didn't go to the gate because it's in a corner and I didn't want to get me and my mare cornered. The horse that was loose is a known kicker - as in: go up to a horse and kick them, not just "you are too close to my bum" kicker.
I just thought of something: maybe I should try and keep a jump in between us? But that would mean constant movement on my part which might further excite a "wheeeee" horse and prolong the incident?