Backmarket review: Missing refund

I purchased a Samsung Flip phone, order number [protected].

After 6 months the phone cracked across the fold.

I returned it as it was within the 1 year warranty period.

The reseller and Backmarket accepted it was defective and unrepairable and issued a refund.

This refund never arrived.

On this basis I am assuming that the refund has got lost somewhere as it have been almost 3 weeks since they stated they would refund.

Whatever I say to Customer Support they just keep telling me that the refund was issued and to contact my bank, my credit card company has no record of a refund being issued.

(A previous refund for another product was received 2 days after the refund was issued)