Sorry, yet another post in a row, but finally thrilled to say it got to run with other rigs for the first time.
There it is at the back of the queue. Ran around solidly for a little while, but then steering trouble struck. Would fully lock in one direction but not revert back at all. A brains trust got together and worked out that the servo arms were not quite in the right place (that's because I'm a bumnut who skim reads important bits). A few minutes of resetting that bit and... much better! All good again. Another run with a borrowed trailer and, yeah, I call that a win.