I'm not sure why you want them to fill out the contact form first? They then need to re-enter their name address email etc again on the mandate form... It's a bit confusing for the customer?Is it so that you know who has signed up?Here is mine >> www.iconicwindowcleaning.co.uk/directdebitAndy
If you use Contact Form 7 then you can get the contact form to forward customers to your gocardless signup page after they press send/submitJust add the following text to the additional settings sectionon_sent_ok: "location = 'add gocardless link here';"