Now that I have started using both WAN ports at the same time, the router seems to be randomly assingning the ports for outgoing connections - so far so good.

I have hit a problem at a particular web site which is constantly asking me to log in again and again. This is due to the outgoing IP address changing between pages.

I have set up a load-balance rule to force it onto WAN1 but surely the router should ensure that the same route is maintained once a connection has been made to a particular site?