OK thanks, the redirect was the answer.
I created a Welcome topic with the following redirect to the
Index for the Help Module:
<meta HTTP-EQUIV=refresh
CONTENT="3;URL=Index_to_Help_Module.htm">
The Welcome topic is not in the TOC, so is not accessible to
the user.
The number in the redirect is the time it takes before the
redirect kicks in.
So now I've got a one-off welcome message that tells the
customer how this help file was created *just * for him - I'm
becoming smarmy enough to consider a career in politics!