Was the item small enough to fit through the letterbox? Current guidelines allow postmen to sign for the package only when they have confirmed the identity of the person. System sucks for sure. They should be asking for ID if they cannot identify the correct house.
Royal Mail will have the GPS co-ordinates of where he signed it so should be able to track it down if it went to another house. Contact Royal Mail support and explain the situation. Ask SurplueStore if they have a picture of the package. I take pictures of every single package before I send it out as a pre-caution, then tomorrow try and chat to your local postie, show him a picture of the package. SurplueStore can also try contacting RM business support.
What annoys me is that the technology has been around for years to avoid problems like this and yet RM still won't implement it. A simple photo of the persons front door when posting signed-for packages would solve this issue. DPD do this