What led me to the solution was this post (11 years old) -
http://data.agaric.com/special-characters-database-password-cause-error-1045-using-mysql-via-command-line. I don't know what even made me think "Special characters might be causing the problem"
Overnight I did some more tests. The partial removal of excel viewer still causes problems even with simple passwords. Using the removal tool and ccleaner to completely clean out any left over .dlls, registry entries etc allows the installation to complete successfully.
Then removal of the ; in passwords then allows the connection to MySQL once the installation has been completed successfully.
Like I said - what are the chances I'd hit two issues in one install!?