Hello,

There is a frequent cause of confusion with Direct Debit.
<snip>
Notably what does NOT happen ever is that invoices which already
exist are suddenly submitted by Direct Debit.
<snip>
a) As soon as a mandate is authorised, just charge all existing
   invoices immediately

b) As soon as the mandate is authorised, if the customer has
   existing invoices that are unpaid, there is a very noticeable
   message on the screen like:
<snip>

I'd suggest the customer has a choice of either in this circumstance, but is required to explicitly choose.  To clarify - I'd suggest that, if a customer has outstanding invoices when signing up for DD, they will be forced to select one of two radio buttons before submitting, for example:

You have existing outstanding invoices and must choose one of the two options below:

• I authorise you to automatically take payment for currently outstanding invoices once the Direct Debit has been set up and (legalese version of: accept responsibility for any fees incurred if this leads to insufficient funds for this or any other payments from my account); OR
  
• I do not authorise you to take automatic payment for currently outstanding invoices.  I will make payment via alternate means or manually authorise payment once the Direct Debit is set up.  I understand that failure to pay will lead to a suspension of service.
  

This would not be shown if they did not have an outstanding debt.  This covers you and hopefully makes it clear to them what, if anything, they will need to do.

Gavin