Just to warn you all, the date stated on the email that Plusnet send out to advise that they are taking money from your account on is not the date the direct debit is initiated. I foolishly thought that by changing my bank account over on the tenth we'd miss the 14th/15th/16th Plusnet said they'd take money on. How silly of me. There then follows a saga of "computer says no" excuses whilst other organisations happily use their direct debits in the subsequent fortnight. Even more amusing when you know Plusnet completely missed taking a payment in the summer and caught up with themselves six weeks later. Where's that MAC code?
While you may have actioned a change in details on the 10th, it takes (IIRC) 7-10 days from Plusnet raising the mandate with the bank and the bank confirming it's setup to Plusnet. And, because you've changed the details a few days before the bill is due, it'll try (and fail) to use the new information before it's ready. Unfortunately, Plusnet still haven't added a warning to this affect on the DD change page despite the number of times this crops up!