A call to BT (who sent the package) elicited the explanation that the courier had taken the parcel back to the depot for the following reason: "problem parcel". She had no idea what this meant, and suggested I call the couriers direct. This I did the following morning (on an 0870 number, I might add - so much for free delivery), and the back office lady at the courier company had no idea what it meant either. She said she'd get the depot to give me a call on Monday morning.
I've called them again this morning to jostle them along a bit, and am waiting for a call on my mobile.