This may not be news to you all, but it was to me yesterday when I rang my water company.
If you are a single person occupant of your property, you get a discounted "special rate" on your water bill - similar to the single occupant discount on council tax.
Of course this is not something the water companies advertise!
Finally, I dont know for certain is this is something all the water boards do. My board is Thames Water and I know that Mid-Kent water also do it as I advised my parents to check.
So it may be worth checking with you water company.