I have had canonical problems previously where Google detects a site as multiple sites, and therefore you don’t get the full impact of the site (as Google see’s this as duplicate content on 2 sites). I have seen some pretty shoddy versions before, but this is by far the best

Options +FollowSymLinks
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} !^www\.siteurl\.co\.uk$
RewriteRule ^(.*)$ http://www.siteurl.co.uk/$1 [R=301,L]