I'd like to create a line which changes colour part way along. Is this possible to do on a bezier curve ( ) or only on objects that have fill?

You can put a gradient on a stoke - use the "Linear gradient" or "Radial gradient" option on the Stroke paint tab of the Fill and Stroke dialog.

Thanks a lot! I note that when the gradient tool is selected, there is a button at the top, underneath the 'undo' button, that enables alteration of stroke (cf. fill) gradient.