Im not sure how to set up WAN2 to my BT hub. Presumably I plug the rj45 between them and enable wan2 but what other settings do I need.
I guess I must make sure the BT hub is set to a differenet lan network address range so its DHCP can grant an IP address to the wan2 port of the 2820. So is it just a matter of setting the dynamic ip address settings?
Anything else?

what you are doing will work , you need to set wan2 to be dhcp client

any reason why you are not plugging your adsl line directly into the 2820 and so not sending traffic via two routers ?

Im waiting for my second bt line to come online. Setting up existing as backup.

When you have both lines working you may want to do a seach on the forum for Load Balancing so you can make traffic go out the different WAN ports rather than just using it as a redundant fail over.