I'm not sure if this will help anyone out but it is certainly something i never knew and wish i had a long time ago.......
I have a water meter and frankly the bills have been really taking their toll on us(over £100 per month).
so i bit the bullet and rang the water board and was thrilled to find out that if you have a child/adult with a disability you qualify to be on the social tarrif which caps your bill at £449 per year regardless of how much water you use.
They are sending out forms and there are stipulations about qualifying but she said that if you get childrens tax credits then you should be fine.
I'm not sure if they do this in your area's but if you are on a water meter then get on to your water board and find out!!