Can someone give me some help with the following? If the user presses
Shift-F1 (or chooses Help, What's This) with a built-in menu option or
toolbar button, they get context-sensitive help; with a custom option/button
they get "Custom ...." which would be great to be able to edit.

I've been looking at the maze that is MS HTML Help and I guess it's to do
with that: I've managed to assign topic ids, numeric ids, html and chm files
so that I can get custom help on UDFs but it would be great if I could also
get context sensitive help for custom menus and toolbars. I've got to the
stage of putting together header and text files for a text pop-up but it's
the calling of these items of help that I'm finding difficult to find any
documentation on: perversely, the "help" for this application seems pretty
much impenetrable!