Thanks MAry Anne, I worked it out and it is quite easy once you know how

MArty, I have used Back Button for focus like forever now
You can focus and meter different areas, you can prefocus on a spot and not have to worry about focus changing when you half press the shutter button to meter, you can leave the camera in continuous AF mode and have both Continuous and Single AF, press teh button and release for single, press and hold for continuous