I work in commercial property water bills , so following may not be true. Most of our sites have water meters, that give a value for water usage . Sewerage or waste water is determined by leasable value , and we have a company who challenges this for us as often too high.
Had a big back charge recently , so think 400x yours where a water meter was not in place for some reason . We fixed this and got a new bill for 25% of so by insisting a water meter was fitted , monitored for a few months and then a new bill based on that.
May help