Disappointing customer service

I made an online purchase on 06/06 which was cancelled by Builders on 10/07, due to lack of stock, and my payment was refunded.

Hoping the item would become available and to be notified as soon as it does, I again placed an online order on 11/07. This order was also later cancelled by Builders on 23/07. I received an email stating that my money would be refunded within 3-5 working days. I followed up on 05/08 as the transaction hasn't been reversed yet. Received a response from Builders on 08/08, stating that the transaction has been reversed and I should "go into the bank and speak to them with regards to your money". I then requested a reference number or something that I can present to my bank so that no one's time is wasted but to date I haven't received any response to this.

I would like to know why it is such a mission to get refunded for goods I haven't received and how it has become my responsibility to prove that Builders Warehouse has returned the money that is due to me.