Apache yes. I thought it might be .htacces but the site has been running for 3 months with no issues. I've pasted the relevant .htaccess lines below - it's unedited from the initial MODX install. Thanks for taking a look.
# Rewrite www.domain.com -> domain.com -- used with SEO Strict URLs plugin
#RewriteCond %{HTTP_HOST} .
#RewriteCond %{HTTP_HOST} !^example-domain-please-change\.com [NC]
#RewriteRule (.*)
http://example-domain-please-change.com/$1 [R=301,L]
#
# or for the opposite domain.com -> www.domain.com use the following
# DO NOT USE BOTH
#
#RewriteCond %{HTTP_HOST} .
#RewriteCond %{HTTP_HOST} !^www\.example-domain-please-change\.com [NC]
#RewriteRule (.*)
http://www.example-domain-please-change.com/$1 [R=301,L]
# Rewrite secure requests properly to prevent SSL cert warnings, e.g. prevent
#
https://www.domain.com when your cert only allows
https://secure.domain.com
#RewriteCond %{SERVER_PORT} !^443
#RewriteRule (.*)
https://example-domain-please-change.com/$1 [R=301,L]
# The Friendly URLs part
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?q=$1 [L,QSA]