Thanks for your reply -
Problem is one can not access preferences if one has not set up any accounts.
Second problem is that the 'default reader' option in prefs is for selecting a mail client (i.e. Thunderbird) rather than a web based email URL.
I did find I can delete the email app itself (not possible with 10.12 and later AFAIK) however this only stops the email app from opening up.
I think I need to explore the possibility changing the browser setitngs as they all seem to use the OSX mail app by default.