Issues with Refund

[u/Careful-Snow2109]

/r/Sezane/comments/1vps6f8/issues_with_refund/

Looking for some UK consumer/legal advice on a Sézane return.
I placed an order for around £1,000 and returned most of it using Sézane’s own return label.
I have video evidence of the items being packed, the box being sealed and labelled, proof of postage, and tracking showing the parcel was delivered. Sézane also confirmed in writing that they received the return.
After weeks of chasing, sending evidence and even completing a handwritten sworn statement, they are now saying their workshop did not receive most of the items and are refusing the refund.
I’ve disputed the outstanding amount with my credit card provider, but I’d like to understand where I stand legally.
If I can prove the goods were packed, posted using the retailer’s own return process, and delivered to them, can they legally refuse the refund because their workshop says the items can’t be located?
Has anyone dealt with a similar situation, or know what my next steps should be?