Happy days - the dock tank chassis works. Balanced on my rolling road it's thrashing away like a good 'un. I expect that with all the joints in the waggly bits things will be a bit tight and some serious running in will be required. This will also show up any problems - in the middle section of the video the valve rod is moving up and down too much because the reversing link has become detached. Once re-soldered normal service was resumed.
The best news is that with the chassis working I can get on with the body. Our show is next week and I'd like to get enough done so I can test the model properly on a layout.