In constructing or maintaining any of the authorised works the authorised undertaker may—
(a)deviate laterally from the lines or situations shown on the Parliamentary plans within the limits of deviation for that work shown on those plans; and
(b)deviate vertically from the levels shown on the Parliamentary sections—
(i)to any extent not exceeding 3 metres upwards; and
(ii)to any extent downwards as may be necessary or expedient.