Help needed- Spark Energy & Past Due Credit Solutions problems

I today received a letter from Past Due Credit Solutions stating that I owe money to Spark Energy Supply Limited - In Administration. This is not the case, I paid this bill at the time I received it via the Spark Energy App. I have a bank statement to prove this. I am aware that Spark Energy went into administration on the 28/11/18 and this bill was issued around that time and paid on 12/12/18. So I called Past Due straight away. After explaining this several times Past due stated they would check with the administrators and if they confirm this they will not contact me again. They also stated that despite the debt being passed to them My credit rating would not be affected. The person I spoke to however did not seem confident on her answers and it seemed that someone else was listening to the call and providing answers. She also had to continue checking with her manager on what to do. I have two questions really and hopefully someone can help.
1) Will my credit rating be affected? I’m proud to say that I have a perfect credit rating and wish to stay that way. I have never had any outstanding debt.
2) Should I seek confirmation in writing that the debt is no longer and was in actual fact an error? If so who do I contact? Also is there anyone I can complain to?
Any help would be appreciated.