Two months notice.
After that you need the courts to evict them if they decide to 'squat'.
I wouldn't worry about the guy being a bouncer, in my experience I have always known friends of friends who are bigger uglier and more psychotic than the people I'm dealing with - most of them are happy to do things for kicks!!!!
I would worry more about the condition of the property once they have left, which one months deposit probably won't rectify.
You always have the criminal damage charges you could bring, but that relies on not only our wonderful police force actually doing anything, but the lunatics that run our justice system siding with you....
I think you have to weigh up whether the loss of a few trees is worth having your property [potentially] ruined, and let things be until the contract runs out. You've no guarantee though that even at the end of the period they will leave peacefully and quietly....