Car insurance trying to scam me?

My policy with go skippy ended in may. I opted out of their auto renewal and they sent an email back confirming this. Just this week ive got an email from them saying I owe an outstanding ballance of £402.92 and they have cancelled my policy.

I got back to them via email explaining how this must be a mistake as I opted out of auto renewal and have not taken a new policy out with them. I sent them proof of previous interactions with them.

Just now ive got an email saying "It has come to our attention that the amount stated in the previous letter was miscalculated. As a result, there is an outstanding balance of £402.92"

I have no idea how this could be the case as i was paying monthly for my premium and paid right up until the last payment ( It was 100 odd pounds a month).

What are my rights here?

Im already in a deep financial hole and am in a DMP. So I could really do without this. Any advice would be greatly appreciated.