6.—(1) In constructing or maintaining works 1a to 1d, 3b, 4b, 5b, 7, 10b, 11b, 12b, 14b, 15b, 16b and 20, the undertaker may deviate—
(a)laterally from the lines, situations or positioning of the authorised development shown or indicated on the works plans to the extent of the limits of deviation shown on the works plans; and
(b)vertically from the levels of the authorised development shown on the sections to any extent—
(i)not exceeding 3 metres upwards; or
(ii)downwards as may be found to be necessary or convenient.