Just a final follow up post.

Managed to contact GSP support, and obtained a local Fedex tracking number that was different from the two numbers that were provided by eBay in the order tracking details.  The two numbers provided by eBay in the order tracking were the USPS tracking number (to Erland, KY), and the Pitney Bowes tracking number (which is of no earthly use at all).

I'm not sure why eBay don't just provide the local Fedex tracking number too, it would have saved their customer support a few phone calls from me.

At any rate, the story ends happily.  The local Fedex tracking number ended up at the loading dock at work, complete with the identity of the person who signed for it. It transpired the item was misdelivered in the workplace, so we managed to recover it.