Hi,
The words statutory instrument and all information in relation to this and council tax can be found at the opsi website.
A link is here.
You can also quote that the council is bound by contract with their agents the Bailiff companies - therefor ultimately responsible for their behaviour and actions when acting on their behalf.
Often the councils themselves do not know the laws or the fee scales.
I would in my letter, include references and attachments if you can that they can refer to, to verify your issues about the charges.
The opsi site above is great for information and is full of acts and statutory instruments.
Quote the Fraud Act of 2006. - link is here, which shows that money obtained by deception is theft. Add that you would like to think they do not condone theft.
Hope it helps.