Stick a redirect into your .htaccess file, something like this:
Code:
RewriteEngine on
RewriteCond %{HTTP_HOST} ^forum.liverpoolway\.co.uk [nc]
RewriteRule ^(.*)$ http://www.liverpoolway.co.uk/forum/$1 [R=301,L]
Probably best at the top of whatever's already in the .htaccess file, and of course make sure you back up the old one first in case this doesn't work on your server.
Basically anyone who tries to get in via the "forum.liv.." way will be redirected to the other way. And it's sends out a 301 code meaning Google and so on should update their links too.