The provisions of this section shall have effect for ascertaining for the purposes of section 19 of this Act the grossr value of a hereditament in cases where it falls to be ascertained by reference to the rent payable in respect of that or some other hereditament (hereafter in this section referred to as the " standard hereditament ") and either or both of the following conditions are fulfilled, that is to say—
Where the rent of the standard hereditament is partly attributable to the provision by the landlord of such services, the sum falling to be deducted from that rent for the said purpose as being the amount attributable to the provision of those services shall not include any amount in respect of—
Where the tenant of the standard hereditament, in addition to the rent—
the rent shall for the purpose of ascertaining gross value be treated as increased by the amount of the payments or other contributions made by the tenant or, where those amounts vary from time to time, by a sum which on a proper estimate equals the average annual amount so paid or contributed.
Nothing in subsection (3) of this section shall be taken to prejudice any right to make a deduction from the rent of a hereditament, for the purpose of ascertaining gross value, in respect of services provided by the landlord or other matters.
Any reference in the foregoing provisions of this section to premises includes a reference to any plant or machinery which by virtue of section 21 of this Act is treated as part of those premises for rating purposes or would be so treated if those premises were a rateable hereditament.