From what the OP said, it was well packaged and bigger than an envelope slot.
On top of that, the house has had many open days and viewings since then with the agency and the landlord doing work so any hope of it being there is extremely slim. I'd need to check when back in UK as to when I catch someone viewing the place.
Where you in the country when the delivery was "made"??
Open days/viewings?? So in effect there has been loads of people in and out of the property and anyone of those people could have lifted the parcel since it was "delivered"
I just don't see how this can be anything can come back on the sender here..... RM saying delivered, this has been confirmed by Postal Review, gps data supports that the postie was at least in the street, buyer unable to check local delivery depot himself as not currently in the country, also confirms numerous people have been in and out of the property over the last 3 weeks. How can any of this come back on the seller here?