Thanks once again Lynne - I'm closing in on the solution.
I'll try to explain my problem differently incase that helps.
I used
this article to include the WYSIWYG editor in my custom template. Using the method in that guide the editor is included by using {vb:raw messagearea} - which works fine for posting.
What I'm struggling is using the WYSIWYG editor to edit existing messages.
I've had a good search around the templates & functions but work out where to insert my value="
x"