Receiving IPv6 stateless configuration

I'm either being really stupid or...

I'm trying to have an interface configure itself using SLAAC. I'm reading through and am I to understand that setting proto dhcpv6 will then accept RAs from the server? Is this the best way to do stateless ipv6 configuration?

Having asked that, this is what I'm trying to configure:

and it seems to never receive an address (other devices on the network receive an address from the server running with this configuration:
I'm curious what I'm doing wrong here.

Also, there is an option in the LUCI interface that looks like it enables SLAAC but instead toggles a config item "

Which seems to toggle a delegate. Any idea what this does? delegate doesn't seem to be a documented network flag.