Cannot install Norton on Mac (Avira won't uninstall)

I have just purchased NortonSecurity for Mac but I am unable to install it and get the error message "we have detected incompatible software installed on your computer.  Please uninstall Avira Antivirus and try again".  I have followed Avira's instructions of uninstalling using their app under the Utilities section (and restarting before trying to install Norton).  I have also tried re-installing Avira and installing again.  I still get the error.  It seems to me that Avira is malware in itself!

Can anyone offer advise on how to clear out Avira or force the Norton installation?

Thank you - Adrian

(Mac OSX El Captain 10.11.12)