Sainsbury’s credit card owe ME money

I cancelled my credit card with Sainsbury’s mid March and received a confirmation letter Dated 5 April acknowledging this and stating they owed me £485.78. 2 weeks later I contacted them saying I had not received it... now numerous complaints to customer services then to the chief executives to no avail... various excuses... firstly I was told it had been paid ... er not in my account- I was then told by a customer service advisor that other people had had similar problems- then each day I’ve had phone calls asking if the money was back in my account?? Then finally last week I emailed the CEO of Sainsbury’s and the CEO of Sainsbury’s bank - funnily enough the executive person I spoke to was the same as customer service person?? I still don’t have my money 6 weeks later? They said it was paid to me but as can be seen my my bank account it has not!! How can I get this money back sooner rather than later whilst they ‘investigate ‘ it and can I request interest on amount they owe me and even request equivalent of late payment fees which they charge if I had of been late?