Where an amalgamation takes place which involves the division of an existing water undertaking into separate parts the cumulo rateable value pertaining to each part shall, for the years referred to in paragraphs 9 to 11 above, be determined in such manner as may be directed by the Secretary of State, and in those paragraphs any reference to a separate undertaking shall be construed as including a reference to the part comprised in the amalgamated undertaking.