As an engineer trained to work on large diesel engined generators as well as vehicles, I feel qualified to say that if you had a diesel car that consumed half a tank of fuel whilst covering under 15 miles, it had either a very small tank or a serious fault. Even doing serious (and slow) off-road work, a diesel Land Rover would do well over 20 MPG.
Current car: Mini Countryman EV in very shiny black!
Previous car: MG HS Exclusive in nice shiny black.