My wife noticed a hissing noise from the right rear tyre, and it became obvious that it was that tyre that had a puncture - however it must have only just happened, as it wasn't yet flat at all. After a bit of investigation with the help of a flashlight, we found the culprit - what appeared to be a screw or bolt in the tyre. As it was already late, I left it until today to sort it out - I ran the tyre into a local place to see if it could be repaired, as the screw/bolt had gone in straight in the center of the tyre, and not on either side, so I was hoping the tyre itself and outer walls were largely intact.
It turned out it was irreparable, and this is the reason why:

What you see in that picture is a bolt, around four or five inches long, the kind that are used in door handles. It wasn't obvious from the outside how big it was, but with the tyre off of the wheel, you can see just how big an item it was. Somehow it went in dead straight, and considering the speed I was traveling at when I picked it up (around 35 mph), it was fairly lucky that no further damage was done. It was also lucky that we weren't too far from home. However what that bolt was doing in the middle of the road, and how exactly it got wedged the way it did, I don't know - the guys at the local tyre place seemed fairly amazed by it too!