Hi
There is no set time for Direct Debits to be processed if you have signed an open mandate, which most of the 35 million people who use DD have.
However, there is a way to control your DD's so you never face 'unpaids' or bank charges.
The simple way is to issue a Mandate change. Anyone can make one up but the Originators (those issuing mandates in the first place) make it virtually impossible. You can write a mandate form in any way you like as it is YOUR account, YOUR money and your payment.
See
http://a2q.co.uk/ for some help.
I used to runa DD scheme so there is not much I don't know about DD.
Regards
AM