In order to complete the model I had to create the middle 'foot' and 'leg', borrowing some geometry previously created. The middle 'foot' is slightly smaller than the others in order for it to fit inside the body.
Once this was finished, I visited the setup room inside Modo and set up locators. They show up as circles in the screen grab. These act as anchors for the different parts of the geometry, so that when I rotate the legs back - for example - the body and everything else in the hierarchy follows along as expected.
When they were all in place I set about posing Artoo into his laid back trundling along mode.
Finally, I exported the model as an OBJ, imported into Keyshot, set up some materials, and produced the following render.
Click for full size
Now I have to tackle my most dreaded job - UVing the model in order to paint it in Substance Painter.