I know that currently you can choose either not to automatically accept a direct debit and then do so manually within 5 days; I also understand that an automatically accepted one that is presented while not enough funds are available will then be put on hold for 5 days and can be manually accepted within that time.
How about if this functionality could be tweaked a little bit so that direct debits that are presented when no funds are available will be booked automatically as soon as sufficient money is in the account (within 5 days). That would take care of cases where an expected payment is delayed a little bit, but remove the danger that one forgets to manually accept it.