The speed reading is not working and the digital display no longer reads. First thought was an earth fault but there is a strong 13V at the connectors and the back lights still come on. How do I decide if this is a dead speedo unit or something external? I could buy another unit, but that would be an expensive mistake if the fault were still there.

Check the cables on the drive (behind the handbrake on the transfer box).

Thanks, I will. If it were just the speed reading that was not working I would have changed the green transducer as a matter of course, but the digital readout has also failed. As the feed for the two things is through different connectors it is causing me to consider that the entire unit is dead.