As from 1st April 1974 any local statutory provision to which this Part of this Schedule applies and which immediately before that date applies to an area which on that date becomes comprised in one or more water authority areas shall have effect, so far as relates to functions which on that date become exercisable by a water authority and to things done or falling to be done in the exercise of those functions, as if for any reference to the body by whom the functions were exercisable immediately before that date there were substituted a reference to that water authority, or where the former area becomes comprised in two or more water authority areas, the water authority for the water authority area in or as respects which that thing falls to be done.