Experian adding accounts to your credit that u dont have

I recently registered with clearscore to check my credit score, so i found out that i have an Rcs Auto account that are in arrears, been sending them e mails regarding this false account on my credit report, called Rcs they say i dont have an account with them so they cant give me any proof of an account that does not exist, so Experian say i must give them proof that i dont have an account with them, so now how is it my responsibility to convince them of an account they added to my credit, and now its a negative also because its in arrears. So can someone please tell me how to resolve this issues, because they are useless in fixing there own problems.