I'm with TCL on this one. Contact your local councillor.
A few years ago, I was getting a 100% reduction on my Council Tax bill (as I was in receipt of just a small pension plus Pension Credit). When my 65th birthday arrived, I then got my State Pension (but lost my Pension Credit), thus increasing my income. Well before that date I wrote to our local Council Tax department to advise them of the forthcoming change. I was unsure as to whether I'd still be eligible for at least some reduction in my Council Tax (as my total income was still quite low) but I was delighted when they continued to give me the full 100% reduction.
Right at the end of the tax year though, they realised they'd made a mistake and (without even bothering to enclose a covering letter to explain things to me) they sent me a revised Council Tax bill. It was for the full year's payment (with no discount) and, because of when it was sent out, there was no option to pay it in instalments. So I was presented with a bill for over a thousand quid straight out of the blue!
I wrote to my local councillor, saying that I felt it was unreasonable that I should have to pay such a big bill, which I'd had no prior notice about, all in one go. She agreed and made contact with their head of finance. The result was that the council agreed to completely write off the bill, thus saving me over a thousand quid ;-)
So contacting a local councillor really can pay dividends!