A repeater for a wireless network is known as a wireless access point.
You need an access point that supports WDS (Wireless Disbtribution System). Make sure that it will support the same type of wireless encryption that you have set in the BT Home Hub (eg, WEP, WPA, WPA2). You will need to enter your BT Home Hub's encryption type (WEP, WPA or WPA2), wireless encryption key, and SSID into the access point.
Then you need to go into your BT Home Hub's configuration (located at
http://bthomehub.home) and go to the wireless section in Advanced, and find a link called something like Scan for wireless access points. When it finds your access point, tick it, enable WDS on the same page, and your wireless coverage should now be extended.