There hasn't been a knock at the door, so I decided to check in at http://track.royalmail.com/. Imagine my surprise when I saw this:
Their website suggests:
If your item has been delivered to a residential address but you have not received it, please first check with family or neighbours to see if they signed for it. If you still cannot locate your item, contact Customer Services on 08457 740 740 or complete the enquiry form online and we will investigate the situation.Sounds very dodgy to me. Someone trying to meet their targets by faking deliveries?
In most cases we will also provide an electronic Proof of Delivery (ePOD) on items that have been signed for. This will show the related Track and Trace number of the item, the time of delivery and most importantly, the printed and signed name of the person who received it.
UPDATE: I called Customer Services, who were monumentally unhelpful. They assured me that the delivery had been made at 12:30, but said it could take several days for them to track down the signature. The best they could suggest was to contact the local delivery office.
Thankfully I didn't have to do that, as a couple of minutes after I got off the phone, a man arrived to actually make the delivery!