Ah, the "rental car full" syndrome.... happens almost always.
Discovered the answer in the latest Skoda Octavia. There is a display in the trip computer, if you cycle through it, for "miles since last refuelling". I showed the guys in Bristol North this, and explained how the car appeared to have done 21 miles when I collected it. Since the nearest fuel stations are about a mile away, someone was taking the proverbial.. at least 21 miles in an Octavia TDi is only about half a gallon.