Where it appears to a local authority that a private supply is to be or is being used for the first time (or for the first time after being out of use for a period of twelve months or more), the authority shall, for the purpose of initially classifying the supply under regulation 9 or 10, estimate the number of persons supplied with water, and the average daily volume of water supplied, in relation to the current year rather than by reference to the year prior to it.