Cancellation of an order from their side and huge delay in processing a refund

I made an online purchase on 12/06/20 which was successful, waited for notification to collect the items, instead after 2 weeks I received the notification that my order was cancelled and I'll be refunded, there was no reason for cancellation beforehand, I had to make an enquiry regarding that ,that's when they told me that the items were out of stock, I was required to furnish them with my banking details or either opt for a voucher, I did both, later I was sent notification that they want ID number for them to proceed with the payment, I responded and gave them my id no on 13/08/20.

Poor service ever, it's not a lot of money but I want it back.