Right first of all I will say Don't bother doing Direct Debit if you haven't already
The reason I say this is that last year, every time me or my friends were meant to have money taken out by this method, it never went and the Uni complained even though it was their fault. I reset up the DD 3 or 5 times and it still didn't work.
I then had to pay each installment by cheque. To do this, you can either use the address on the back of any invoices they send you, or take it in person as I do, to the Finance Office on the Ground Floor of the Main Building.