DipTrace bottom layer pad shape
As this PCB is getting printed onto acetate then going in an etch tank I have figured out a workaround. I previewed the PCB, and selected Layers > Show > All layers, and Layers > Current > Top. I then deselected all the objects in the Objects menu that weren't going to print. This left me with exactly what I wanted, albeit in a rather botched manner. This method is pretty useless if you are doing anything other than single layer DIY etching without silk screens etc. Hopefully this answer may be of some use to someone.