A big thank-you to Rob Bowker, Richard Polt, "rn" and to Adwoa for their suggestions and encouragement with my efforts to get my "new" Imperial Model D Portable Typewriter working.
I think I have located the source of the problem, as the videos below will indicate. Anyone watching the videos might be able to spot the difference from a working Imperial Model B (black) and a non-working Model D (green).
I have removed the keyboards-typebaskets on the Model D and my Model B to see where the differences in the mechanisms are. And the inner workings of the Model B and the Model D do differ somewhat, as can be seen from the photos below. One obvious thing is that the B has a rounded, fairly wide plate under the typebasket which activates the pawl and the carriage movement.
Neither has a pawl to engage with the escapement rack, as such - not a round one, anyway. It is more like a three-toothed tomahawk head. Is there a spring missing from the D (green)? The spring on the Model B can be clearly see pulling the "pawl" forward.