Incorrect savings impacting Universal Credit

Back in 2022 my Uncle applied for UC and declared he has £8k in savings which is untrue. In fact he has no savings but as he has mental health issues he completed the form on his own and incorrectly. As his mental health has worsened I’ve been asked to help (I’m one of his nephews) and speaking to UC team they advised we need to provide proof of his bank account and savings, by providing a statement for 2022. It’s already a challenge as my Uncle can’t recall 15 minutes ago so for 2022 it’s impossible for him to recall. I have the last 4 as punt digits and I’ve gone through the Lost Account app and all the major banks have said they don’t have anything. He wouldn’t have an online account back then as he doesn’t understand the internet. So his UC has been set, at a reduced level, since 2022 because he completed the form wrong and UC won’t change anything without a bank statement. They don’t have a copy of the recording or any information on what the bank details provided at the time so I would like to know if anyone else has had this challenge and how they resolved it please. Thanks.