20.—(1) This paragraph applies to a relevant supply contract if the billing periods are prescribed in the supply contract.
(2) Where paragraph (1) applies, the customer must, as soon as reasonably practicable after the scheme introduction date or, if later, the date the supply contract is entered into, notify each lower-tier provider of each benefit calculation period which starts or ends in a scheme period.
(3) Where paragraph (1) does not apply, the supplier must, as soon as reasonably practicable after it has decided on a billing period, notify the customer of the corresponding benefit calculation period.
(4) Each provider which is notified by a higher-tier provider of a benefit calculation period must, as soon as reasonably practicable, notify each lower-tier provider of that period.