Head to Sales > Invoices.
Click on an unpaid invoice number.
Under the Payment history table click on the mandate request link below the Active GoCardless mandates header.
You will now be re-directed to the Tools > Integrations > GoCardless > Mandates menu.
The customer will be pre-filled in the drop-down box click on the Request mandate button to email the customer.
All requested mandates will appear in the Requested mandates table on the Tools > GoCardless > Mandates menu. You have the ability to cancel or re-send the request in the Options drop-down list.