If you wish to setup a Direct Debit for your Pay Monthly Hosting plan you simply need to change your Payment Method on the next invoice that you receive.
Please do this as soon as you can after receiving the invoice as it usually takes a few days to setup the Direct Debit Mandate with your Bank.
When you view your invoice you will see that it is set for a Payment Method of Credit Card or Paypal and the PayPal Checkout Button will be shown at the top right hand corner.
Simply click on the dropdown under Payment Method and change this to Direct Debit.
You will then see the Setup Mandate for Payment Button. Clicking on this will take you to the screens to setup your Direct Debit and all future invoices will be paid by Direct Debit directly from your bank account.