26.—(1) Regulations 24(1) and 25(1) do not apply to a silo or slurry storage system—
(a)which, before 1 March 1991, was being used for the purpose of making silage or storing slurry,
(b)where it was not used before 1 March 1991 for that purpose, it was constructed before that date for such use, or
(c)in relation to which—
(i)a contract for its construction, substantial enlargement or substantial reconstruction was entered into before 1 March 1991, or
(ii)such work was commenced before that date, and
in either case the work was completed before 1 September 1991.