Fedex trying to claim import duty for returned goods

shaun_from_Africa

well when the OP ordered they were no doubt told they were responsible for an duties due

yes, clearly stated on the Diamonds International website.

NOTE: Additional fees for duty and taxes may apply and are the responsibility of the buyer. We reserve the right to collect these additional fees from you in the event that FedEx is unable to.

The problem you have aiden is that Fedex paid the required duty when they imported your ring, and rightly expect you to pay this (and their fee for doing so).
As it now stands, the return of the ring to the USA and the reclaiming of the duty and VAT is no longer anything to do with them.

Whet you should do is to pay Fedex what you owe, then contact HMRC and get them to send you the relevant forms for you to try to reclaim the duties paid.
You will have to explain to them that the goods were not kept in the UK but were returned to the seller (hopefully you will be able to provide proof of this with the shipping documents and possibly a credit card statement showing the refund).