In case anyone else has similar redirect needs, here’s the code from the .htaccess file that solved the problem :
Based on an example by netprophet.
# For full documentation and other suggested options, please see
# http://svn.modxcms.com/docs/display/MODx096/Friendly+URL+Solutions
# including for unexpected logouts in multi-server/cloud environments
# and especially for the first three commented out rules
#php_flag register_globals Off
#AddDefaultCharset utf-8
#php_value date.timezone Europe/Moscow
Options +FollowSymlinks
RewriteEngine On
RewriteBase /
# Fix Apache internal dummy connections from breaking [(site_url)] cache
RewriteCond %{HTTP_USER_AGENT} ^.*internal\ dummy\ connection.*$ [NC]
RewriteRule .* - [F,L]
# Rewrite domain.com -> www.domain.com -- used with SEO Strict URLs plugin
RewriteCond %{HTTP_HOST} .
RewriteCond %{HTTP_HOST} !^www\.your-domain\.co\.uk [NC]
RewriteRule ^(.*) http://www.your-domain.co.uk/$1 [R=301,L]
# Exclude /assets and /manager directories from rewrite rules
RewriteRule ^(manager|assets) - [L]
# For Friendly URLs
RewriteCond %{REQUEST_URI} !^/(prod1|company|prod_detail|enquire|print_product_page).asp
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?q=$1 [L,QSA]
RewriteCond %{QUERY_STRING} ^ID=217$
RewriteRule ^prod1.asp$ /a-confident-plumber-in-weeks.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=201$
RewriteRule ^prod1.asp$ /? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=81$
RewriteRule ^prod1.asp$ /about-us.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=214$
RewriteRule ^prod1.asp$ /your-plumbing-questions-answered.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=226$
RewriteRule ^prod1.asp$ /part-p-defined-course.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=224$
RewriteRule ^prod1.asp$ /unvented-hot-water.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=234$
RewriteRule ^prod1.asp$ /city-and-guilds-6129-level-2-technical-certificate-in-plumbing.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=225$
RewriteRule ^prod1.asp$ /water-regulations.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=235$
RewriteRule ^prod1.asp$ /funding.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=223$
RewriteRule ^prod1.asp$ /our-plumbing-courses.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=220$
RewriteRule ^prod1.asp$ /our-plumbing-courses.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=218$
RewriteRule ^prod1.asp$ /our-plumbing-courses.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=221$
RewriteRule ^prod1.asp$ /our-plumbing-courses.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=215$
RewriteRule ^prod1.asp$ /? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=210$
RewriteRule ^prod1.asp$ /our-plumbing-courses.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^ID=222$
RewriteRule ^prod1.asp$ /part-p-defined-course.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^prod_id=51$
RewriteRule ^company.asp$ /privacy-policy.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^prod_id=8161$
RewriteRule ^prod_detail.asp$ /our-plumbing-courses.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^id=8161$
RewriteRule ^enquire.asp$ /contact-us.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^id=8165$
RewriteRule ^enquire.asp$ /contact-us.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^id=8162$
RewriteRule ^enquire.asp$ /contact-us.html? [R=301,L]
RewriteCond %{QUERY_STRING} ^id=8166$
RewriteRule ^print_product_page.asp$ /part-p-defined-course.html? [R=301,L]