Hi mark, I fill up 6 times a week. Basically the sewerage company just look at your water consumption to get their charges, because there is no way of you proving that you are not dumping your water down their drains, and I have argued with snotty nosed cow at the sewerage company, they charge you accordingly.
I was talking to a lady (not snotty nosed!) recently about the sub meter, but to be honest all that is going to tell them is exactly how much water you are using to fill your tanks, i can tell them that because their is a meter on the van! but if you have a sub meter fitted its down to you to pay the installation costs.
In my opinion my water costs me slightly less than £500 per year, not even £2.00 a day! I know of alot of businesses who would chop off an arm for overheads like that especially when the water is a main component of the business.
But if anyone has a better solution I will gladly listen!!
greg