Thanks Uhelp, but I was after 'how' to send the HTML rather than how the message should be formatted.
Typically, I searched yesterday and couldn't find anything - and then found this within an hour this morning:
mutt -e "set content_type=text/html"
the -e from the mutt man page:
-e command
Specify a configuration command to be run after processing of initialization files.
This appears to solve it with my first test message. Hopefully, I can now lay out my message with Libreoffice or similar and just run a script to control the mailing.
Solution taken from:
http://www.unix.com/red-hat/123764-s...t-command.html