Ok here's my thoughts - though others have raised plenty of good points already.
Before starting on a letter like this, try not to focus
too much on your legal rights imo. There's two kinds of arguments you can use to persuade this guy: legal and moral, and the latter is far more powerful I find, in this kind of situation.
The thing is, even if (legally speaking) you're 100% right, and you can prove all the components of your case, there's nearly always going to be a way for him to screw you. He could just say family came back to town, or the house needs urgent repairs, anything like that - and there's basically shit all you can do about it. Either that, or he'll just call your bluff and let you file a disputes tribunal case or whatever.. And if he returns your deposit, you might have trouble proving 'damages' etc... I'm probably labouring this point a bit, but bringing up the law in detail at your first point of contact will tend to get peoples' back up. One thing that legalese does is show you're serious, but imo, a similar affect is achieved just by speaking coherently and structuring the letter well, which you clearly won't have a problem with.
So yeah, I would cut out a bunch of that legal research and focus on the actual harm that he is causing you first. Keep the rhetoric emotionally neutral, and say why you don't think his intended actions are fair etc. If that doesn't work, then bring in the law for Round 2