late estate account not found

after my mother passed away i opened a late estate account with Absa, i was given cheque book to use, i used that account until i no longer had funds in it . so a week ago i claimed at oldmutual and they needed that account so they can deposit the money into it , now my problem is that Absa is saying they want an account number and i don't have it as i lost the cheque book years ago . i have given them all the documents but still they cant help me . my question is , if the bank itself cannot help me with the account number, how will i be able to get it. it really doesn't make sense, clearly there should be a way to assist me. Absa please do something.