Yes take your confirmation receipt for the payment with you. Also you will need the print off of your online appointment. Make sure the applicants name and passport number is correct. You will need to get a pass into the building on the ground floor. Then take the lift to the 6th floor and away you go. It's a stressful time efore but it gets better once it's handed in. The reality dawns it's out of your control then. But do the preparation right, check and re-check before you hand it in. Make sure the i's are dotted and the t's are crossed. That's what we did, then hopefully there is nothing to hold up the application.
Provide a reasonable level of evidence. Photos together, flight bookings, e-mails, YM history. All that is good stuff. Spread the evidence throuhout the time you have know each other.
Good luck with the application.