There's every chance the buyer is being dishonest but even if they're being honest and a Royal Mail employee stole the item the buyer doesn't really have any incentive to do anything as they are covered by eBay's Money Back Guarantee.

You really need to protect yourself here by uploading a return label - if it isn't used you will be credited for it. This statement from the buyer is concerning:

"Ok mate iv just spoken to them they are investigating with Royal Mail they told me to wait and to call them on the 4th and go from there"

Assuming the buyer opened the case yesterday (Sunday 31st May) they can escalate the case on or after Wednesday 3rd June. As things stand if/when that happens the buyer will be refunded at your expense; that is why you need to upload a return label. I'm guessing this is the ~£300 item so you really need to follow the instructions in the case - return cases are automated and you are guaranteed to lose by default if you don't upload a return label.