Having got the Courier IMAP server up and with all my mail migrated onto the dedicated mail server zone, I decided to put a webmail interface on the front.
Having previously played with a few - Squirrelmail, Horde and Roundcube, I concluded that Roundcube was the one I wanted to use. The interface is very clean and it's simple.
I used Blastwave to setup the required components - Apache, PHP, MySQL, edited the httpd.conf to include PHP support, configured the database and we were off.
The final step was to configure sendmail so that I could use Roundcube to send emails as well. This proved more difficult than it should as sendmail was failing to resolve post.demon.co.uk (my upstream mail relay). This was fixed by specifying Demon's DNS servers in /etc/resolv.conf.
So now I have access to my email via Thunderbird (on Linux), Mail.app (on the Mac) and Roundcube (web browser). At some point I may investigate a commandline MUA for when I SSH into the network, but this will do for now.