Thanks for all the input; I think I'm nearly there but it is still not routing any traffic over IPv6.
I set up as above, including the virtual IP as a3sx, and finally the WAN_DHCP6 has come up and is green (it wouldn't without the virtual IP). Amazing, never worked before. I took the address from configuring 'none' on WAN ip6 and seeing the loopback address after reboot (where does this come from??) it starts fe80::
My devices on the LAN are getting IP6 addresses and I can see leases on 'DHCPv6 Leases' status screen.
My devices are getting IPv6 addresses starting with 2002:89dc... etc, could this be based on my delegated prefix? (Where do I see the prefix I got?)
Yet when I open browser and do an IPv6 test all IPv6 tests fail. If I ping 'google.com' over ipv6 on diagnostics on the webUI it fails as well.
Feels like it's close but there is still something wrong.
Pfsense+ 23.01
If somebody would be able to look at my screenshare I'd send them money for a beer in the pub!
thanks B