I know Ben very well. He's a wonderful dog, full of bounce. He's been doing this virtually from Day 1, and I think it will be difficult to change him. A course of obedience lessons might do it, or send him away to be trained (Charlie?) I have a breed that hunts at 150 to 200 yards each side of the gun, and they have to be let off only in areas where I know they are safe - the Dam and peppermill. I'm lucky I have an acre they can run in most of the day, as well as large outdoor grass runs.
My sister's Lab used to run off when he knew they were getting near the car but would come back during the walk for a treat if they called him, so what they did was park the car in a safer area and walk him away from it, then walk him back to it, then away again - both on and off lead, and giving loads and loads of tasty titbits - big bits chicken was a favourite, then keep calling him back throughout the walk, putting him on lead and treating, walk for a bit, then let him off again. He is a big lab too and is too strong for Helen to keep him on a flexi all the time.(He pulls and they've never managed to stop him)
Not much help I'm afraid, maybe back to basic obedience, treating him as a puppy might work. Good luck. See you soon. May pop in today, will be out later.