Provided that any alterations or extensions of a scheme which are, in the opinion of the Board or, as the case may be, the Generating Company, minor in character may be made without preparing a supplementary scheme:
Provided further that where any alteration or extension of the nature referred to in the first proviso is made in respect of a scheme concurred in by the Authority, details of such alteration or extension shall be intimated to the Authority as soon as may be after such alteration or extension is made.