The speedo on my square dash was not working. It is well known that this is often due to one of the internal gears moving out of mesh, and I thought it might be useful for anyone else with the same problem to see exactly what it looks like. So this is the situation as found ...
... see how the red cog is not engaged with the white worm drive, whereas when fixed, it's more like ...
In my case it wasn't that the red cog had moved on its spindle, but that the spindle itself had moved; I don't know if that's normally the case.