Council tax reduction - help needed asap

[Bird1983]

have had a visit from a council fraud investigator to talk about a recent application i made for housing benefit. I called him back as i was not in when he visited and told him that i did now not need to claim as i am now working and have proof that the claim is closed. He said he also wanted to talk to me about the whereabouts of my now husband for the time i have claimed the 25% single persons discount. He is coming to our home next week to talk about this. I notified the council when he moved in back in 2015, but it appears that the records have not been updated. I do not have proof that i notified them as it was by telephone, i did however get the bill then updated in my married name and my husband was added to the electoral roll.
Up until we got back together he was living with his sister and paying her rent and she is happy to confirm this. Why is he visiting our home and what do I need to do to prepare for this?i am also 8 months pregnant and suffering with ill health. If we have made a mistake ( i 100% called to notify the council of him moving in) then we are happy to pay any shortfall but the stress of this all is really not good for my health or the health of my baby.
I also didn't mention that prior to my husband moving in i was in arrears with my c.tax which are now cleared, and i have a direct debit now set so I haven't ever had a fixed amount to pay as i have always been paying the shortfall off otherwise i may have noticed this error.