What you’re saying is of course true - the problem with DDs is that the payment is already in process a long time before it’s actually payable but the DD Guarantee goes further and specifically states:
You can cancel a Direct Debit at any time by simply contacting your bank or building society.
Most banks action these requests by cancelling the underlying DD, stopping any payment requests if they are far enough in advance to do so and recalling any payments on the day of payment if it’s too late for that (net effect being the payment occurs and is then reversed later the same day). Natwest Group, for whatever reason, don’t do that last part - they warn this is the case and advise you to contact them to recall, which I did. I was told to contact again on the day of payment, which again I did (but you can already see how cumbersome this is now).
Unfortunately the message I sent on the day of payment during office hours wasn’t picked up until too late. They then tried to fob me off with indemnity claims (not applicable) and offers to send me to their credit card team who’d sort out a refund (cool, except it was a Virgin Money credit card that had been double-paid, not a Natwest one).
I’m satisfied with the outcome but moving forward I won’t be using Natwest Group for DDs except for a couple of savings account deposits each month to satisfy the Reward Account criteria.