Thanks for your post @benjig07 and apologies for the delayed response.
If you pay by direct debit and want your payment leaving your bank on or around the 15th, I'd recommend setting your bill date as the 8th because there's a 5 working day advance notice period for direct debit payments.
If you pay by card a payment will typically leave your card within 24 hours of your bill date.
I hope this helps.
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