(5) Where an interest in land which has been acquired by agreement before the commence ment of this Act by a person authorised by virtue of any enactment to acquire it compulsorily is an interest in a hereditament or part of a hereditament which has sustained war damage then if—

1. (a) the conditions specified in sub-para graphs (1) (a) and (1) (b) of this paragraph are satisfied in relation thereto; and
2. (b) the person to whom the purchase price is payable in respect of the acquisition of the interest in question would, if the interest had been acquired compulsorily, have been entitled to any such supplement as is mentioned in sub-paragraph (1) (c) of this paragraph;

the amount of the purchase price payable in respect of the acquisition shall be reduced by such sum as may be appropriate, not exceeding the amount by which the value payment is increased as is mentioned in sub-paragraph (I) (b) of this paragraph, or the amount of the supplement which would have been payable as aforesaid, whichever is the less; and sub paragraphs (2), (3) and (4) of this paragraph shall apply in relation to the reduction required by virtue of this sub-paragraph to be made in the purchase price as if for any reference in those sub-paragraphs to the compulsory acquisition of an interest in land or to the amount of the compensation payable in respect of that acquisition there were substituted respectively a reference to the acquisition of an interest in land by agreement arid to the purchase price payable in respect of that acquisition.