Its confidence mate.
Do your research. Know exactly what your paying per cube in. And out.
Then know exactly your figures for cubes used and down the drain. And more importantly-the ones that weren't put down the drain. (your pure+any waste that went on your grass etc)
Don't ask them. TELL them.
'Hi, this is Matt from 21 Whereever Road, customer number 123456789, I'm ringing to give you the figures from my not returned to the sewer sub meter'
'We haven't got a record of you registering your sub-meter'
'You should do. I called on 15/06/2014 to register. My reading then was xxx and my sub meter reading is xxx'
'Im sorry we can't do anything about that, full price is payable'
'Please transfer me to your line manager'
'Im sorr...'
'Transfer me to your line manager please'
etc etc
Thing is Matt-you need to convince them that you know what you're talking about.
That you know your figures.
And that you aren't going anywhere until you get your bill sorted properly.
You need to be absolutely crystal on your figures.
Perhaps have them written down in front of you while you're on the phone.
As soon as they think you're bluffing you'll get the 'computer says no...' response.
If it aint gone down the drain, you ain't paying'. End of.
One warning.
I had this conversation 2 years ago with my water people (Anglian) they said i owed them £1,100.
We settled it over the phone within an hour or so...
And they still took the money on DD
I had to wait '30-60 days'!
Got it back about 5 weeks later. No compo or anything.
apparently you only get compensation for them taking money if you can prove it caused hardship.
Which it didn't so i didn't get any money extra.
They are gits.
But you can beat them.