We rent our house on a rolling tenancy. We paid, I seem to recall, something like two and a half month's rent up front as a deposit. When we leave, the return of that deposit is conditional on our leaving the house in good condition and on all rent being paid up to date.
If you took a deposit when your tenants moved in, then you would be entitled to deduct what you're owed in the same way. If you didn't then you'll probably have to go to court if they don't pay up. You'd have to weigh up the cost of a solicitor against what you're owed.
Taking a bond/deposit is essential in an agreement like this, for both sides. If you haven't done so this time, make sure you do so with your next tenants.