Cheers for the link, it wasn’t quite what I needed but it got me to the right menu.
what I needed was:
Menu - setup list - wing tail mix - V-tail [act]
(This turns on the mixing for rudder and elevator)
Then
disable rudder stick:
Menu- MIX1 - rudder -> rudder up -100% down -100%
Enable rudder control using aileron stick.:
Menu - MIX2 aileron -> rudder up 100% down 100%
Hope this helps someone in the future.
Ps I could have use the Elevon mix as per the video link, but my other TX (Devo 7e) has it setup to mix the aileron and Rudd so I wanted both TX to be setup the same.