Okay, i've been through the FAQ's and documentation and set everything I could, so I contacted BT, who gave this reply:
"Dear Mr Brown,
Many thanks for your email regarding your problems using shoutcast software.
BT Internet do not block any ports, including 8000. The modem sharing
programs which you mention in your email are used localy - on users
computers, rather than on BT Internets servers.
I would recommend you ask a variety of different people to connect to your
server, if they all receive the same ''Connection Refused'' error please get
back to us and we will investigate further.
If you require further assistance, please do not hesitate to contact us."
No further forward then, so its not being blocked by my ISP, its not being blocked/masked by my linux box, as otherwise it would do the SAME on my other dial-up connections.