Sorry IAP but you are totally wrong. You cannot have a Direct Debit between two people - only between people and companies or companies and companies.
http://www.bacs.co.uk/BPSL/directdebit/general public/faqs/DC_public_FAQ.htm#Q16
"Q. Who makes sure that the organisations collecting money are reputable?
A. All organisations using the Direct Debit system are sponsored into the Scheme by their bank or building society. They are checked for integrity, sound financial standing and administrative capability before being permitted to offer Direct Debit to their customers"
I have just been on the NatWest site and logged in as I to am an online customer - you have set up a standing order, not a Direct Debit (unless of course you signed up to the Direct Debit Guarantee - a condition of setting up DDs)