"but when it comes to preparing a custom Table of Contents, it will not allow me to modify the installed styles"


I realised I used the wrong word. This problem will not allow me to format the ToC levels to suit the layout I want. For instance, the built in formats do not have hanging indents, but I like that so I add them in from ToC 2 onwards. I can modify ToC 1 but all the levels after I click "Modify" then "Format" I get the message "This style name already exists or is reserved for a built-in style"