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Where a sum is payable to a relevant authority by any person by virtue of this Act in respect of the expenses incurred by the authority or by virtue of section 36 of the Local Government Act 1974 in respect of establishment charges related to such expenses or by virtue of section 193 of the Local Government (Scotland) Act 1947 in respect of general expenses related to such expenses, then—
the authority and that person may agree that the sum ... shall be paid in instalments; and
the authority shall be entitled to receive from that person interest on the sum ... , or on such portion of it as is for the time being unpaid, at such reasonable rate or rates as the authority may determine.
In the application of this section to Scotland, for the references to a water authority there shall be substituted references to SEPA.