Solved!
Just a note to anyone looking to do the same thing in the future...
I just had to change the appropriate template for the /etc/tcprules/tcp.smtp. Modifiing the first line to appear as follows: 127.0.0.1:allow,RELAYCLIENT="",QMAILQUEUE="/usr/bin/qmail-queue.amavis"
Then run tcprules tcp.smtp.cdb tcp.smtp.temp < tcp.smtp
And your away.
Remeber, simply changing /etc/tcprules/tcp.smtp is not enough as this change will be lost when you modify the "Email" section of the server-manager. Modify the appropriate template.
Cheers,
Tim