Billing irregular payments via direct debit

[#1]

Having has the run-around from Customer Service, I thought it worth finding if others had experienced a similar issue.
I have a direct debit mandate in force which has happily takes payments from PN for many years, including the one for this month. In December I imported an additional domain. The hosting fee was not taken from PN’s next debit but remained outstanding. Out of the blue today I receive a Final Reminder telling me, inter alia, that my account has already been deactivated, that the debt recovery vultures would shortly be circling and so on. It starts “Despite our previous letter…” ….but there was no previous letter – at least none popped through my post-box. There was also no reminder email (did it go into the spam bucket?) despite this being a requirement of PN’s T&Cs.
Customer Service tell me that irregular fees of this nature are not taken by direct debit even if there is a mandate in force, but only by credit or debit card. When asked to point me to the T&C section which specified this, there was an embarrassed silence (it may, of course, be hidden there somewhere). The same response came from the Billing group when, to vitiate deactivation, I made a hurried debit card payment (this is a Business account and an outage would be Really Bad News)
The debit mandate I authorised is, as far I I know (and this was long time ago!), a general mandate: as far as I know, PN can debit me what they like subject to a retraction guaranteed by the bank is the debit is invalid.
Could anyone – PN or otherwise – elucidate please?

[#2]

PlusNets billing systems have never been able to issue differing DD mandates each month when different products are involved (domain reg in your case), P&P for routers, change of packages where a payment is due etc. They can deal with additional bandwidth allowance changes as it is all part of the same product.
It's not in the T&C as that does not cover types of billing methods only the requirement to pay.
Its is however worrying that you did not receive warning about non-payment. Do you collect email from your postmaster mailbox?
Also have you looked back through your closed tickets as this kind of thing is normally recorded there.

[#3]

Thanks Peter - but this is a hosting charge (i.e. is part of the service), not a registration.
PN's own definition is: "Service" means Services provided by PlusNet to the Customer under this Agreement and includes all or any part of the range of Internet connectivity, subscriptions, content, services and applications provided to you by us.
I would deem 'hosting' to be included in the above. Perhaps PN do not.
PN's payment clause is: 13.2 When you register for the Service … we will ask you to select a payment method …. Following activation of the Service, Charges shall be payable in advance monthly or annually unless any other payment method has been agreed in writing between PlusNet and the Customer or as detailed on any accepted Customer Order. Charges for Equipment ordered by you are payable by Credit or Debit Card at the time the order is placed.
[Strictly, these do not apply to business customers, but they are the only definitions I can find!]

Which anyway begs the question of how I am to know that the payment will not be taken via my debit mandate. My mandate is for charges to my account. If PN cannot debit me this way, how am I to know this?
It not (apparently) in the T&Cs and not (apparently) in the 'please host this domain name' dialogue panels. In fact, not (apparently) anywhere - but I wait to be corrected if it lurks in small print.   
The T&Cs state that direct debits always take precedence over credit or debit cards (if the latter are extant), except where payments for supplied equipment are concerned.
After my 'discussions' with Customer Service and with Billing today , two retrospective reminders were posted within minutes on my Closed Tickets (and I was emailed). There were no previous closed tickets concerning this problem. 

[#4]

Regarding the "missing" letter: have you checked your address on your account details are correct? (I remember someone in the last few months who's address had reverted to one they moved away from years ago)

[#5]

My address is correct (in Account Details).
And the Final Reminder found me!

[#6]

Hi,
Can you send me a PM with the domain name, and any copies of emails you *DID* receive?
I'll pick this up when I'm back in the office tomorrow and get any problems raised for investigation by our development team.