To use more than one file for a sketch It turns out that you need to do a little house keeping ....
There doesn't seem to be a way to change the order of the tabs once they are open.
They open in file name order, alphabetically.
If the compiler encounters a variable used in the first file before it has been declared in the second file you get a compilation error.
The point being that the files seem to be assembled physically with the leftmost tab ant the top.
My project is called HMI.
I have an HMI directory with HMI.ino as the main file, this contains the declarations section
also in the directory are :-
HMI_01_Functions General functions, I will probably split this as the list gets bigger.
HMI_02_Setup setup() code.
HMI_03_Main loop() code.
HMI_04_Scratch Space to play without messing up the formatting.
When the project is opened in Arduino 1.0.1 the tabs are in numerical order and code compiles and runs just fine.
I hope some of you will find this useful