Thanks Rupert. My 'trick' if you can call it that for clay renders is to use an Image Based Lighting node. I surround the model with a modified sphere that has a flat plane for the car to sit on, and apply a standard lambert to all the geometry and sphere (color pure white). Then once you've done all the standard things setting up the Final Gather, go to Render Globals>Mental Ray Tab>Image Based Lighting and hit create. A new window will pop up in the attribute editor, and you can attach your HDRI image to the light node. I've been using a sunset by the beach type HDRI, with a soft orange/yellow feel to it. That way, you can dictate the 'feel' of the clay because it picks up subtle shades of the HDRI and bleeds the color. Doing this saves you having to actually create an environment sphere and attaching the image that way.