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—
(a) the rent of the standard hereditament is partly attributable to the provision by the landlord of services in relation to that hereditament (including the repair, maintenance or insurance of premises not forming part of that hereditament); or
(b) the tenant, in addition to the rent, contributes towards the cost of any such services.
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—
(a) any profit made, or which might be expected to be made, by the landlord in providing those services ;
(b) the cost of repairs to, and maintenance and insurance of, premises not forming part of that hereditament.
Where the tenant of the standard hereditament, in addition to the rent—
(a) makes payments to the landlord in consideration of the landlord undertaking to provide any such services in relation to that hereditament; or
(b) otherwise contributes (directly or indirectly and whether in pursuance of an undertaking to do so or not) to the cost of repairing, maintaining or insuring other premises not forming part of that hereditament but belonging to or occupied by the landlord, being premises which the landlord has not undertaken to repair, maintain or insure, as the case may be,
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.