You never heard of Jeremy Clarkson thinking this then?
He was so confident that people couldn't do buggar all with his account number and sort code that he wrote them in his column for the sun.
1 week later, in his next column, he explained how he was miserably wrong and people took about £5000 out the bank account.. and of course the bank wouldn't do anything about it because he posted it lol.
Account NO + Sort Code are the only two things you need to set up a direct debit payment. The amount... is up to you.
That's half true. The bank are responsible for returning the funds under the DD scheme.