Contact the seller again to explain that you still have not received your item, do this through the ebay messaging service so that they can see what has been said and when. If the seller doesn't respond, you have the option to escalate it to a claim for a refund. The seller may choose to send you another if that is possible. It is their responsibility to ensure you receive your item, and claim off Royal Mail if you don't. Ebay will almost certainly find in your favour if it goes to a claim. Bear in mind Royal Mail will not consider an item "lost" until 15 working days after posting, so there is still time for it to arrive. I once had a parcel turn up over two months late.