When I booked mine I was unaware that i would have to pay in advance, so I said I would call them back a few days later with a CC, I explained that it was good old mum who was paying and she was on a business trip at the time.
They were fnie about that, but said that it would have to paid for in advance. They kept the booking for me, I paid about a week before the examination and everyone was happy.
They are really nice down there, and happy to help.
Good luck