Graham - This is the fix (not my work - copied from the net)
1. Within the Settings->Wi-Fi Networks menu on your device, choose BTOpenzone.
2. When the device has associated with the BTOpenzone SSID, click the arrow next to the BTOpenzone entry. This shows the detailed settings for the connection.
3. Select the DNS section of the details shown. The DNS section will list 3 IP addresses "10.xx.xx.xx, 192.168.22.22, 192.168.22.23"
4. Scroll back through the list by dragging finger across the screen and then delete the initial 10.xx.xx.xx address.
5. Check the DNS section entry is set to "192.168.22.22, 192.168.22.23" (There should be no space at the beginning of the entry)
6. Return to the main screen and enter the Safari browser. Navigating to any Internet URL will redirect to the BTFON landing page. Follow the instructions to log in and connect to the Internet.
You don't need to worry about deleting that first DNS entry, it won't have any other effects on your connection."