I assume the buyer has gone through Ebay and has started a return case using the Ebay Money Back Guarantee,

The next step is for you to accept the return and provide a label so the buyer can send exactly what they claim they received - insist they return all the original packaging as well.

You should tell the buyer that RM are investigating this already as Special Delivery is a very secure and highly regulated service with little scope for interference in a package.  Keep all messages in the case - you have the chance to add a message via an opened case.

If the buyer then returns the tuna, you must report them to Fraud Online (previously Action Fraud) who will give you a case number.

Then, go back to Ebay and say the buyer has returned a different item, you have reported it and RM are investigating. Give them the Fraud Online case number.

Ebay should then look at all the information they have, including the record of you and the buyer, and make a decision on the case.