You don't need to make a demand for rent.Wouldn't a demand for rent imply that a tenancy is still in place?
Rent remains due until the end of the statutory notice period. Whether the tenants have vacated the property or not is irrelevant.
So the sequence I would recommend, if you are intent on retaining the security deposit, is as follows:
1. Carry out your final inspection of the property and receive the keys.
2. Wait until the end of October and then notify your tenants in writing that you will be retaining their security deposit to cover the unpaid rent.
3. Ready yourself to respond to a complaint to the RTB (if it hasn't already been made at that point)!