This is a strange one. You did presumably clear the cookies as suggested? If this happens again try making the booking in private/incognito mode and avoid auto-fill for the card details (I've had issues making payment online on other websites populating the card details this way). If none of that works try a different network (for example not your home Wi-Fi or mobile) just in case it is a temporary issue with your network provider.

Let us know how you get on, if you ever try them again!