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 Agency and if Network Rail fails to do so, the Agency may make good the same and recover from Network Rail the expense reasonably incurred by it in so doing.