two weeks behind

Rent Arrears

I have a new tenant who agreed to pay by rent by standing order.
After one month when the second payment was due no rent arrived.
For three weeks he has been telling me he had a problem with the bank due to moving address and the payment is now on the way.He has told me this four times.
I notice on the rla tenancy agreement which i am using it says we may repossess if the tanant does not pay rent 14 days after it is due.
What is the best way to threaten this?
Bernard