Hi Lissa, Thanks for the suggestion. Much appreciated. All the best, Ian

Dear Ian, maybe the following solution is of interest to you. I don't know how to do it using DynamicModule, so I used Manipulate instead.

    opts = Association[
       "a" -> {"dog", "cat", "horse"},
       "b" -> {"car", "boat", "plane"},
       "c" -> {"apple", "pear", "banana"}
       ];
    Framed[Labeled[Column[
       {Manipulate[opt2 = {};
         Labeled[CheckboxBar[Dynamic@opt2, opts[opt1]], 
          Style["Select Options", Bold], Top],
         {{opt1, "b", ""}, {"a", "b", "c"}, PopupMenu}, Paneled -> False],
        Dynamic@opt2}], Style["Main Menu", Bold], Top], 
     Background -> LightBlue]