3.—(1) The base liability for the relevant year beginning in 2005 for an altered hereditament shall be found by applying the formula Y x Z, where—
Y is the amount mentioned in sub-paragraph (2), and
Z is the non-domestic rating multiplier for the financial year beginning in 2004.
(2) The amount referred to in sub-paragraph (1) is the amount certified by the appropriate valuation officer as the rateable value which would have been shown for the hereditament for 31st March 2005 in the list on the assumption that—
(a)the hereditament had come into existence on 31st March 2005; and
(b)the matters referred to in paragraph 2(7) of Schedule 6 to the Act as respects the hereditament were as they were assumed to be on the day on which it was shown for the first time in the local list as mentioned in paragraph 2(b) of this Schedule.