Sorry only just coming back to this. Yes I use HAProxy on the front 
Do I need to do anything special on the Discourse side to enable IPV6?
On the HAProxy side I believe I have changed everything I need to, specifically:
bind *:80
to
bind :::80 v4v6
and
bind *:443 ssl crt /etc/haproxy/certs/
to
bind :::443 v4v6 ssl crt /etc/haproxy/certs/
On http://ready.chair6.net everything passes except for:
IPv6 Connectivity - FAIL - Could not connect (to site) on port 443 over IPv6.
Any idea what I might be missing?