necessary for such compliance and may recover any expenditure incurred by it in so doing from  Network Rail.  (4) In the event of any dispute as to the reasonableness of any requirement of a notice served  under sub-paragraph (2), the drainage authority must not except in a case of an emergency  exercise the powers conferred by sub-paragraph (3) until the dispute has been finally determined.  (5) This paragraph does not apply to—  21.  Subject to paragraph 23, if by reason of the construction of any specified work or of the  failure of any such work the efficiency of any drainage work for flood defence purposes is  impaired, or that drainage work is otherwise damaged, such impairment or damage must be made  good by Network Rail to the reasonable satisfaction of the drainage authority and if Network Rail  fails to do so, the drainage authority may make good the same and recover from Network Rail the  expense incurred by it in so doing.  22. —(1) Network Rail must take all such measures as may be reasonably practicable to prevent  any interruption of the free passage of fish in the fishery during the construction of any specified  work.  (2) If by reason of—  damage to a fishery is caused, or the Agency has reason to expect that such damage may be  caused, the drainage authority may serve notice on Network Rail requiring it to take such steps as  may be reasonably practicable to make good the damage, or, as the case may be, to protect the  fishery against such damage.  (3) Subject to paragraph 23, if within such time as may be reasonably practicable for that  purpose after the receipt of written notice from the Agency of any damage or expected damage to  a fishery, Network Rail fails to take such steps as are described in sub-paragraph (2), the drainage  authority may take those steps and may recover from Network Rail the expense reasonably  incurred by it in doing so.  (4) Subject to paragraph 23, in any case where immediate action by the Agency is reasonably  required in order to secure that the risk of damage to the fishery is avoided or reduced, the  drainage authority may take such steps as are reasonable for the purpose, and may recover from  Network Rail the reasonable cost of so doing provided that notice specifying those steps is served  on Network Rail as soon as reasonably practicable after the Agency has taken, or commenced to  take, the steps specified in the notice.  23.  Nothing in paragraphs 19(4), 20(3), 21, 22(3) and (4) authorises the drainage authority to  execute works on or affecting an operational railway forming part of Network Rail’s network  without the prior consent in writing of Network Rail such consent not to be unreasonably withheld  or delayed.  24.  Network Rail must indemnify the drainage authority in respect of all costs, charges and  expenses which the drainage authority may reasonably incur or have to pay or which it may  sustain—  25. —(1) Without affecting the other provisions of this Part of this Schedule, Network Rail must  indemnify the drainage authority from all claims, demands, proceedings, costs, charges, penalties,  (a) drainage works which are vested in the drainage authority, or which the drainage  authority or another person is liable to maintain and is not precluded by the powers of the  Order from doing so; and  (b) any obstruction of a drainage work for the purpose of a work or operation authorised by  this Order and carried out in accordance with the provisions of this Part of this Schedule.  (a) the construction of any specified work; or  (b) the failure of any such work,  (a) in the examination or approval of plans under this Part of this Schedule; and  (b) in the inspection of the construction of the specified works or any protective works  required by the drainage authority under this Part of this Schedule.  102