Thanks for your question - I am Sam and I am one of the UK tax experts here on Just Answer.
Just contact them either via the telephone or by letter - advising that you no longer wish to claim single persons discount and advsie them effective from which date you wish this withdrawn, which ideally should be from 10 years ago.
The council will then recalculate what you owe along with interest charges
If you just make the declaration from now - they are more likely to ask for evidence as you are doing this instead of filling in the claim form, which will alert more suspicion then you having done this mid year, and I am sure you can appreciate for me to encourage you to declare any date than the correct date, would see me not doing my job properly
Let me know if I can assist further