21. If at any time after the completion of a specified work, not being a work vested in the Trust, the Trust gives notice to the undertaker informing it that it reasonably considers that the state of maintenance of the work appears to be such that the work is causing or likely to cause detriment, the undertaker must, on receipt of such notice, take such steps as may be reasonably necessary to put the work in such state of maintenance as not to cause such detriment.