how do i un ctrl-U

I now have a complicated file as my starter - new file.
how do i get an empty file as my starting file?
I pressed ctrl U instead of alt U

CTRL+U saves the current setup as the default. You could set it up the way you want Blender to open, and press CTRL+U.

…or delete the file .B.blend, and on next launch Blender will open with the original default settings.

thanks for your help

If you want to revert to “factory defaults”, either delete the .B.blend file or do File>Load Factory Settings