*** BREXIT UPDATE ***
Following confirmation from both Customs and ferry providers that their systems would now accommodate the option, we have completed a test run using multiple trucks under a single carnet. The test went as well as could be expected and the trucks have arrived back in Dover, the drivers have discharged the Carnet, and are running for home.
We experienced a couple of hours delay on the way out, but that was due to some confusion over procedure rather than a procedural failure. There was a little bit of shrugging and arm waving from French Customs on the return, but nothing too serious - It appears that they are in the process of changing their procedures in Calais and this kind of threw them a curve ball.
One interesting point of note. The Customs Clearance Status is now displayed on Screens on the ferry. Our Drivers winced on the outbound crossing when they went Orange, BUT they came off the ferry, followed direction to the documentation check, got the carnet stamped within 10 minutes and were on their way out of the port very promptly.