after much faffing around and a number of useless call to BT I managed to find the answer.
Once you change the local IP address you need to update the DHCP settings - I suspect you already tried this (like me) and found it can't be changed either. Bad UI design if you ask me as it's not obvious how this setting works!
Basically, the trick is to select 'Custom' in the DHCP options list. This will allow you to change the DHCP address range to one that matches you new IP. Now, when you click 'Save' the new settings will be applied. It seems that if the DHCP address range is unchanged the settings are not saved (for good reason).
Hope that helps.