I assume that your BT broadband modem is "Also" a WiFi router
AND that your Time Capsule is attached to the BT router via an ethernet cable, where the ethernet cable attaches to the Time Capsule's WAN port. Since your Time Capsule has a 192.168.x.x address, I think this is correct.
If my assumptions are correct, then you should configure the Time Capsule for "Bridge" mode. That is to say you do NOT want to create a "New" network, you want to bridge the existing network through the Time Capsule.
Next, you want to give the Time Capsule the same network name (that is to stay the same WiFi SSID name) as the BT network, AND this is very important, the exact same WiFi Password.
This will create a "Roaming" network, which is what companies use to give WiFi coverage over a very large area via multiple WIFi access points.
Next assign your Time Capsule to different WiFi channels than what the BT WiFi router is using. For the 2.4GHz 802.11g radio you want to be at least 5 channels away from what the BT WiFi 2.4GHz 802.11g radio is using to avoid any interferences. In the U.S. we are restricted to 2.4GHz channels 1-11 (not sure if the UK is just 1-11 or if they allow 1-13). And when assigning 2.4GHz channels, look at the channels your neighbors are using and try to stay 5 channels away from the neighbors that have strong signals (which would be the one that live closes to you). You can get utilities to see the other WiFi routers in your neighborhood and their assigned channels, such as iStumbler from someplace like <http://MacUpdate.com>
You could also try (from a Terminal session)
/System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -s
If you do not need a roaming network, then still put your Time Capsule into "Bridge" mode, but now see if you can turn off the WiFi radios of either the BT WiFi and use your Time Capsule for WiFi, or see if there is an option to turn off the Time Capsule radios and use only the BT WiFi.
Message was edited by: BobHarris