In receipt of ESA income related, lost council tax support backdated 5 years

Hi,
Life has been quiet for the last few years, regarding benefits issues.

Five years ago my adult daughter returned home to do a post-graduate course which moved on to a masters finished only last week.
When she moved back I informed housing benefit and a proportion of it was reduced from it to account for her megre earning (part-time work).
With the new assessment council tax reduction was decreased accordingly which meant she had to pay a portion of council tax (Westminster).
Throughout these years our account was reassessed countless times has her wages rose or decreased through the times she had to change jobs due to business closures etc.

Suddenly the council decides I am liable to pay full council tax, therefore losing my council tax support for the past 5 years.
How can this be? When did they make a mistake? Now or five years ago? They effectively are throwing us into financial chaos. if they made the mistake 5 years ago we are being denied the chance to ask for help elsewhere during that time or take steps to deal with it at the time. My daughter has never asked for universal credit while her job hours were low when she could have. We made do and managed to pay all our bills. This seems a bit unfair to me.

Can someone illuminate me on what my position is and what can I do about it now?
My Housing benefit has also been suspended but I'm yet to hear what will happen there.
Thanks for any help in advance

anumao