from what i have read the latest BT home hub firmware update might block your telnet access to the router, i've read you can roll back the firmware but you'll have to look into that yourself (im zen adsl myself)
http://support.steampowered.com/kb_article.php?p_faqid=50
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Zen (UK) Routers
or other ISP customers using a SpeedTouch ST585/ST546/ST536/ST780WL, BT Home Hub, or BeBox router
Please follow this Zen Detail Report with specific instructions on how to resolve this issue if you are using this hardware.
If that does not resolve the issue, then please follow these directions:
*Note: if you are not comfortable with Telnet or editing your router's advanced configuration, we recommend contacting your ISP or router manufacturer for assistance*
1. Click on "Start", then select "Run", type "cmd" and press Enter or click "Ok". This will bring up a command prompt.
2. Type "telnet speedtouch", or if that doesn't work then type "telnet 192.168.1.254".
3. You will be prompted to login to the modem with a username/password. The default is one of the following:
Username: Administrator
Password: None (leave this field blank)
OR:
Username: admin
Password: admin
If neither of those combinations work, please contact your Internet Service Provider for assistance.
4. Once you have logged in, you will be presented with the command line of your modem. This is where you must enter the following commands:
* Type connection bindlist and press enter. This will list connection bindings.
* Look for a line that contains "GAME" and some port numbers. For example: GAME(UDP) udp 27010-27011
* Enter the command: connection unbind application=GAME(UDP) port=27***-27*** and press enter. The port numbers in this command must match the numbers that are listed in your GAME port listing, similar to the example above.
5. When finished, close the command line and launch Steam. Try joining the game server and see if it works correctly. If it does not, please contact your router manufacturer or Internet Service Provider for assistance.