So with a number of hot polyester mixes and testing I've ended up with the following layout for the demist plenum.
In the foreground a dam I needed to add to add after making up a splitter at the rear throwing air in both directions.
In hindsight now perhaps just the dam with the right cut out would have been all that was needed but too late now.
And the glass splitter return before I tidied it all up.
Anyway the interlude between fibreglass hardening sessions allowed me to get on with other work as you will see on my other "miracle build" thread.
Just need to sikaflex the plenum under the dash top and I can then turn my attention to the dash proper and kick panels finally - hooray!!
Thanks for the various suggestions lads!