(b) to secure that its efficiency for flood defence purposes is not impaired and that the risk of  flooding is not otherwise increased,  (a) without unnecessary delay in accordance with the plans approved or settled under this  Part of this Schedule; and  by reason of any specified work.  19. —(1) Subject to sub-paragraph (2), any specified work, and all protective works required by  the drainage authority under paragraph 18, must be constructed—  (2) Network Rail must give to the drainage authority not less than 14 days’ notice in writing of  its intention to commence construction of any specified work and notice in writing of its  completion not later than 7 days after the date on which it is completed.  (3) If any part of a specified work or any protective work required by the drainage authority is  constructed otherwise than in accordance with the requirements of this Schedule, the drainage  authority may by notice in writing require Network Rail at Network Rail’s own expense to comply  with the requirements of this Part of this Schedule or (if Network Rail so elects and the drainage  authority in writing consents, such consent not to be unreasonably withheld or delayed) to remove,  alter or pull down the work and, where removal is required, to restore the site to its former  condition to such extent and within such limits as the drainage authority reasonably requires.  (4) Subject to sub-paragraph (6) and paragraph 23, if within a reasonable period, being not less  than 28 days from the date when a notice under sub-paragraph (3) is served upon Network Rail, it  has failed to begin taking steps to comply with the requirements of the notice and subsequently to  make reasonably expeditious progress towards their implementation, the drainage authority may  execute the works specified in the notice and any expenditure incurred by it in so doing is  recoverable from Network Rail.  (5) In the event of any dispute as to whether sub-paragraph (3) is properly applicable to any  work in respect of which notice has been served under that sub-paragraph, or as to the  reasonableness of any requirement of such a notice, the drainage authority must not except in an  emergency exercise the powers conferred by sub-paragraph (4) until the dispute has been finally  determined.  (6) If by reason of construction of the specified work the Agency’s access to flood defences or  equipment maintained for flood defence purposes is materially obstructed, Network Rail must  provide such alternative means of access to allow the Agency to maintain the flood defence or use  the equipment no less effectively than before the obstruction.  20. —(1) Subject to sub-paragraph (5) Network Rail must from the commencement of the  construction of the specified works maintain in good repair and condition and free from  obstruction any drainage work which is situated within the limits of deviation and on land held by  Network Rail for the purposes of or in connection with the specified works, whether or not the  drainage work is constructed under the powers conferred by this Order or is already in existence.  (2) If any such drainage work which Network Rail is liable to maintain is not maintained to the  reasonable satisfaction of the drainage authority, the drainage authority may by notice in writing  require Network Rail to repair and restore the work, or any part of such work, or (if Network Rail  so elects and the drainage authority in writing consents, such consent not to be unreasonably  withheld or delayed), to remove the work and restore the site to its former condition, to such  extent and within such limits as the drainage authority reasonably requires.  (3) Subject to sub-paragraph (5) and paragraph 23, if, within a reasonable period being not less  than 28 days beginning with the date on which a notice in respect of any drainage work is served  under sub-paragraph (2) on Network Rail, Network Rail has failed to begin taking steps to comply  with the reasonable requirements of the notice and has not subsequently made reasonably  expeditious progress towards their implementation, the drainage authority may do what is  101 and an officer of the drainage authority is entitled to watch and inspect the construction of such  works.  (b) to the reasonable satisfaction of the drainage authority, 