Missing parcel and no interest from Fed Ex at all.

I was ordering from inside UK for a London parcel delivery just over a month ago. The company lost my 16 Kg parcel, telling the sender the parcel had been delivered and signed for by someone using my first name only. I have tried to go through the complaints process and simply get told to go back to the seller. There seems to be no security check or any way to prove to them that I did not receive my parcel and am left between a company that should not be refunding me who sent the parcel and a huge company who just don't care that someone seems to have ****** my food delivery of expensive and bulky items. Shockingly bad customer service, I have dealt with them via email so I have a record of their responses, but mostly get automated reply not relevant to the details I have supp****.