Standing order payment

I have started renting my property out last month. My tenants paid the deposit and 1st months rent upfront. I advised them to set up a standing order for the 13th of every month there after.

Yesterday would be the first payment due, but because it fell on a sunday, it should have gone in my account today.

Nothing is showing, my question is:

How long should I wait before contacting the tenant if it does not go in by close today?

should i text, phone, email or send a letter to them to discuss the payment.

I know it is due today, but they have advised the standing order is set up, so surely it is an automatic credit first thing on the day due?

Any help would be appreciated.

Many thanks