Royal Mail's special delivery terms state that due to the covid situation "we're no longer asking the recipient to sign for items".
As a signature is still required, I imagine that the postman signed it.
Special delivery is a premium service for valuables. It's most unlikely that they will have dumped the parcel on the doorstep, as other services often do.
I think you need to do two things here. First, as has been stated, ask the buyer to open a case under eBay's money back guarantee. With a signature for delivery, eBay should find in your favour. If so, they will block the buyer from leaving negative feedback, or remove any already left.
Secondly, I think you have to take this up with RM on the buyer's behalf. A claim of non-delivery for an item recorded as being delivered by special delivery is very rare. I imagine that RM investigators will wish to look into your buyer's claim. If found to be justified, as this is a fully insured service you should be able to claim full compensation for them.
One of the many shortcomings of eBay's money back guarantee is that they're unlikely to wait for the outcome of RM's investigation.
To keep things friendly, I suggest that you let your buyer know that you will be taking this up with RM on their behalf, regardless of the outcome of their eBay case.
