After reading datasheets and application notes I was still no closer to setting up this chip. I then decided to use three 1K trimpots as voltage dividers across the power rails and pin 4 , 6 and 7 to set the lower , upper and LED brightness. And IT WORKED beautifully, I can adjust the voltages by putting a MM across the relevant pin and adjust the pot until I get the voltage I need. I've even put an LDR across the pin 7 pot and now have an autodimming feature. This seems too easy, am I doing something that will come back to bite me or have I just got lucky doing it this way?