"forget the rent arrears " - err, why? Deduct it from the deposit. If she owes more than the deposit, because of dilapidations etc, then you can go to court to reclaim the remainder (assuming you have a good inventory at start and finish).
If rent owed is more than two months you can issue s8 as well as s21. The fact is though, if she doesn't move out on the day then, yes, you will need to get a court order and instruct bailiffs.
Personally, I'd try to have contact with her during the notice period - talk to her about what needs to be done on the moving out date, meters that will need reading etc, any cleaning that you expect to be done - that way you should be able to gauge if she is really intending to move out on the day or whether she is simply using delaying tactics.