Could you clarify what you mean by "the buyer rerooted the parcel while in transit"?
Did the buyer contact the courier with the tracking number, and persuade them to change the delivery address? I seem to recall that this is possible with some couriers, but not I imagine with Royal Mail.
Under eBay's rules, sellers are required to deliver the item to the address that eBay provides in the order details. That is the address to which they will require evidence of delivery.
I don't think we've heard of this situation before, but if the buyer hijacked the delivery to have it redirected and the courier stuck the label on the wrong parcel I don't see how you can be expected to refund them. Or, for that matter, how eBay can be expected to if it was sent by simple delivery.
It's all a bit complicated, but if this is what the buyer is demanding keep all their messages about this and contact customer support for advice if they open a case under eBay's money back guarantee.
