Thames Water backdate charges up to 6yrs for whole street!

Thames Water supply our water. Wessex Water are responsible for our waste. Thames water charge us for the waste water and pass the money onto Wessex Water on our behalf. Due to some administrative error they have been underchraging the whole street for waste water for a number of years.
Over the past two weeks we've all received three bills from them. Two of them detail credits as they credit all of the waste water charges back to our accounts then close the accounts and re-open a new one backdating the higher wessex charges in our case for 2 years as that was when we moved into the property. We now owe them £169.00 Some of our neighbours have bills over £1000!
When we called them they informed us that they were legally able to backdate charges for 6 yrs.

Has anyone else come across this? Has anyone taken on the water companies and won?