Water tax demand by the authorities before date of purchase of flat

I would like to state that the Flat was purchased by us from LDA (Lucknow Development Authority) and the possession of Flat was handed over in the year 2010 as such the House Tax has been calculated and being deposited from the year 2010. In the year 2010,we received Water Tax Bill from JalKal Vibhag Lucknow demanding Water Tax from the year 2005; in this regard we had submitted application and clarifing that since the Flat was purchased in 2010 and accordingly the House Tax is being paid from 2010 the should be applicable for Water Tax but the Jalkal Vibhag Lucknow is demanding water Tax from the year 2005. As per the terms of Sale Deed (Registry) executed between LDA (seller) and us (buyer), all Taxes and Dues before the date of sale of the Flat has been cleared by LDA, hence outstanding water Tax if any for the period 2005 - 2010 must be claimed from the seller (LDA). Please advice the future course of action.