I used my account boardmans

On 05 September 2015, my fiancee bought a 25l boardmans microwave, a month later it started making a funny sound when activated, we took the appliance back to the store as it was still under warranty. At the store the microwave was tested and we were told that it will be replaced, when they required the receipt we explained that we don't have the till slip as it got damaged when the geyser burst and spilt water into the house but that we had used an account and understood that it can serve as a proof of purchase, we asked for the manager who seemed like the dumbest manager you can ever come across, she said she doesn't have any means of accessing the records to prove we bought it there (its a boardmans brand), she further said signing after every account purchase is just procedure and served no purpose and therefore can't help us, I then told her that the if we were not paying those records would be accessible, she asked that we leave the microwave and she will try her best to send it for repairs, that was in December 2015 until now its still in the store...what is the purpose of the account? Is it not to assist in cases of misfortunes...I sure will spread the word about this store