ISO Range Problem 1D Mk3

I have happily shot 90k+ shots on my 1D Mk3 - but sports shots that stuck within the ISO 400 to 1600 range. At the weekend wanted to shoot at 100 ISO and discovered that I could not get below 200 ISO. Simple - it must be the custom settings. Changed CFN 1-3 to H and L - pressed Apply and tried again. But ISO range still limited to 200 - 3200. [and before the CFN 1-3 change I found that it was set to Disable!]

I guess that I am making some stupid mistake and there is some other setting I have missed - but have re-read the manual and searched the forums without spotting my error. Help!