You are a star!
Works .. with a small caveat.
Being a long term cynic.. been in IT too long.
1) Remove the pseudo subdomain devel.mydomain.com
2) Ran update
3) Added new subdomain devel.mydomain,com
4) Tested devel.mydomain,com - Ok
5) Modified index.php in devel.mydomain.com to make sure i was looking at the right file - all good
6) Made the modes - template and /etc/php/7.0/fpm/pool.d/www.conf
7) Re-tested devel.mydomain.com -
file not found Re Re tested devel.mydomain,com/index.php - ok
9) added index.html in devel.mydomain.com
10) and tested devel.mydomain.com - ok showed content of index.html
11) Added <?php phpinfo(); ?> to index.html - Worked like a charm!!
So Only issue - which I can live with, is that the .html is the default extension and .php must be specified.
Not a problem for this site
Thanks again