Thank you MikeT, tygrys, RayC. There is a path at the rear of the drivers step, if you look under the van, there are plastic blanking plates, which can be removed, drilled to allow cable to pass through and replaced. I also stumbled upon the entry point you mention RayC, that is yet another possibility. In the end, I used the entry point at the rear of the drivers step, since it was slightly larger, and allowed the video cable (with connectors) to pass through more easily. So, to recap:
I took power supply from rear of cigarette lighter, passed wire up drivers side windscreen pillar, into head lining and out to monitor attached to rear view mirror. Video cable and camera power, I led from rear of van, under the floor, entering van at rear of drivers step, then behind trim to join up with video and power from monitor where it emerged from window pillar.
The camera id intended to allow me to "keep an eye" on motorcycle trailer, so, I wanted it "always on" rather than "on in reverse only" hence, taking power from behind cigarette lighter.
Hope that helps somebody out there.