Ok, I figured out what I need to do.
Following both of these links...
http://httpd.apache.org/docs/2.2/bind.html
http://httpd.apache.org/docs/2.2/vhosts ... .html#port
I realised that I had to add the port for Apache to listen out for and then specify it in the virtual host file for that domain.
For example:
Inside /etc/apache2/ports.config
Code:
...
NameVirtualHost 12.34.56.78:80
Listen 80
# Add port you want to use
Listen 8080
...
Inside /etc/apaches2/sites-available/exampledomain.com
Code:
# Spedify the port number
<VirtualHost 12.34.56.78:8080>
...
And don't forget to reload Apache.