At joomlaseo.com, I saw a version of code which is very slightly different.
The difference is the last term in the bottom line.
RewriteEngine On
RewriteCond %{HTTP_HOST} ^site.com [NC]
RewriteRule ^(.*)$ www.site.com/$1 [L,R=301]

Whereas in your version we have this: [R=301,L]

Is the difference material? I know it's possible to break the site if this is done wrong.

But I do want to give it a go.

Thanks again for all the assistance.