I don't get it Deas, I pay my life insurance, my car insurance, my equivalant of the AA by an annual DD, it comes out as one payment. As far as I know that makes them a DD. The amounts change each year, I get notified of the amount and that it will be taken on x date and that's it.
In relation to you point about an arbitrary picking of a date mid month. Why didn't revenue sent the letters in February with a deadline for credit cards in March with the same date as the single debit authority. What is the advantage to revenue to getting some of the money now?