Great thanks.
As a note to anyone who finds this, add a file such as /etc/dnsmasq.d/02-strict.conf, with a single line of strict-order.
Note your preferred DNS should be listed as #2 in the Custom List (ref "strict-order" seems to be working in reverse - #4 by DL6ER)
@DL6ER, would be really great to have this as a check box so PiHole could simply add the directive and then add the servers into the conf file in the logical order.