Because water and sewerage charges stay with the land, any outstanding charges become the responsibility of the new owner at the date of settlement. This is the case whether you or the previous owner used the services charged for on the first bill.
This is usually adjusted at settlement, meaning the price you paid for the property is reduced to cover the seller's portion of our charges. When you purchased this property, it's likely your solicitor or conveyancer arranged a 'special water meter reading' to identify any charges owed by the previous owner.
You can confirm this on the property settlement paperwork provided by your solicitor or conveyancer, or by contacting them directly.