21.—(1) The undertaker shall before commencing construction of any specified work supply to Network Rail proper and sufficient plans of that work for the reasonable approval of the engineer and the specified work shall not be commenced except in accordance with such plans as have been approved in writing by the engineer or settled by arbitration.
(2) The approval of the engineer under sub-paragraph (1) shall not be unreasonably withheld or delayed, and if by the end of the period of 28 days beginning with the date on which such plans have been supplied to Network Rail the engineer has not intimated his disapproval of those plans and the grounds of his disapproval the undertaker may serve upon the engineer written notice requiring the engineer to intimate his approval or disapproval within a further period of 28 days beginning with the date upon which the engineer receives written notice from the undertaker. If by the expiry of the further period of 28 days the engineer has not intimated his approval or disapproval, he shall be deemed to have approved the plans as submitted.
(3) If by the expiry of 28 days beginning with the date on which written notice was served upon the engineer under paragraph (2), Network Rail gives notices to the undertaker that Network Rail desires itself to construct any part of a specified work which in the opinion of the engineer will or may affect the stability of railway property or the safe operation of traffic on the railways of Network Rail then, if the undertaker desires such part of the specified work to be constructed, Network Rail shall construct it with all reasonable dispatch on behalf of and to the reasonable satisfaction of the undertaker in accordance with the plans approved or deemed to be approved or settled under this paragraph and under the supervision (where appropriate and if given) of the undertaker.
(4) When signifying his approval of the plans the engineer may specify any protective works (whether temporary or permanent) which in his opinion should be carried out before the commencement of construction of a specified work to ensure the safety or stability of railway property or the continuation of safe and efficient operation of the railways of Network Rail or the services of operators using the same (including any relocation, decommissioning and removal of works, apparatus and equipment necessitated by a specified work and the comfort and safety of passengers who may be affected by the specified works) and such protective works as may be reasonably necessary for those purposes shall be constructed by Network Rail or by the undertaker, if Network Rail so desires such protective works shall be carried out by the undertaker at its own expense with all reasonable dispatch and the undertaker shall not commence the construction of the specified works until the engineer has notified the undertaker that the protective works have been completed to his reasonable satisfaction.