Payment is taken at around 23:00 on the day of the clean.
Although payment details are required for registration, you are never billed until after your clean has taken place.
Upon registration, or when you update your payment details, you may notice a £30 authorisation charge. This sum is never actually charged, and the charge will disappear from your statement within a few days, at the latest (although usually much sooner). Authorisation charges have the payment reference "HK_Auth"