Confusion about posible council tax reduction claim

Driftingleaf

This might be a little silly to ask, but it is about council tax reduction. For the back story, my mother was severly ill and I was her full time carer and she was getting Attendance Allowance higher rates and I had carer's allowance. My mother had a neurological illness which impacted her mentally too so she had an impairment and couldn't reason with her, she had obsessions, couldn't pay bills etc. My father was alive, but he couldn't be a full time carer with his own health problems. Now, my parents never looked at getting a reduction in council tax for themselves. I don't think they were aware they could have potentionally been entitled to it. It was several years ago now and I only just found out AA could mean a possible reduction. I rang the council concerned, 1 person was from the benefits bit, she said they'd send out a form if I got historical evidence of AA and from the gp practice my mother was with. I was looking at getting phone numbers to get the evidence when the council tax department rang and said no, this was too many years ago and there were 3 adults living there so it couldn't be done. I googled it and a) a website said if the person met the severe mental impairment and b) a carer who does 35 plus hours of care each week is also factored in so that the carer should not be counted too. In that case my mother and I would be disregarded for possible reduction on the bill (disregarded was used on the website). The council tax woman said no, 3 adults living there, so no reduction could be considered. She also said if was 2 weeks ago it could be looked at, but a few years back is too long, but according to another website you can try to claim as far back as when CT was introduced which was 1993. I did live there just to clarify and although my name was not on the bill nobody mentioned this wasn't a reason to get it backdated only it was "too late". Should I try anyway? A form is being sent to me and I was going to put a covering letter. I thought I'd fo my own research and found at least the council tax woman was wrong that 3 adults living there meant no reduction could be given, but my mum had mental impairment and I'd be disregarded as her carer.