I fitted a sub meter to my garage tap where I fill my van,when the bill comes I take a reading from southern waters meter then a reading from my sub meter,then phone southern water and give them both readings they then calculate how much water has not sent been to sewer and then credit me the difference.
I put the waste water from my ro either in my pond or on the garden hence no return to drain
Well worth doing and will save you a few hundred quid a year