Steve, I know yours is dirt cheap but honestly I think your water company has cocked up and you will get a massive bill at the end of the year 
The return to sewer allowance is only a percentage - you don't get something for nothing plus there's the household useage to consider.
Even if you put nothing down the drain and didn't use an ro system - you still pay in excess of £2 for every cubic meter of water.
I have to use an ro - even if my rejection rate was 50-50 which it isn't then this puts my water at £4+ per 1000 litres of pure.
Anyone know how much your average osmosis unit rejects?
Dave I have been on a meter for well over a year now, they have inspected it and we have queried each bill and its still the same. I have the same conversation each time
so if they are wrong that's is not my fault as we have questioned the price and they insist they are right.