Have you got the latest firmware for the hub?
Try selecting 'XBOX Live' as application sharing on the hub, then restart the hub.
You could also try resetting to factory defaults on both the hub and the xbox 360 to see if that cures the problem.
If that fails then the best bet is to ring XBOX 360 Support 0800 587 1102 and select the option for live.
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Tenacious_T
BT Total Broadband Option 3
BT Broadband Talk
BT FON
BT Vision
BT Home Hub v1.5
Downstream 8128 Kbps.......Line Attenuation 19 dB
Upstream 448 Kbps............Line Attenuation 8.5 dB
IP Profile 7150 Kbps...........IP throughput 6917 Kbps