In the case of a water undertaking wholly situated within a rating area the cumulo rateable value for the year 1967-68 or any subsequent year, as ascertained under the foregoing provisions of this Schedule, shall be taken to be the rateable value of that undertaking for that year and the Assessor shall F1give a direction under section 5 of the M1Local Government (Scotland) Act 1975 to the local assessor to enter that value in the valuation roll.