I found it.. within the FH Mod Toolkit.
Thank Jebus.
I have always just used Battlecrap at the final stage to render terrain lightmaps but it wont allow full terrain Large maps so shadow gen becomes a pain. I cant believe I have missed this tool until now.. It's great.
it will generate a custom imlist.dat file and also take the pain out of manual shadow gen, merging and objectlightmap gen.
BF42 Light.exe
grab it here:
BF42_Light.exe-imlist.dat_generator.rar