Thanks guys.... saving to an AVI file was the key message.

When I used Studio previously it kind of renders and burns in one go and I missed the fact that in an intermediate phase it saves as an avi file too. But as I never needed to go and look at that file outside of the Studio program I didn't need to be aware.
I need to experiment with the protype titler to get scrolling titles and the menu function as well, but I'll save that for another day when I am not under so much pressure to complete the project !
I am impressed though... the quality of output on my TV seems better going through the two step Vegas process of "project" to AVI to DVD burn than the all-in-one Studio approach, so many thanks to you both for your patience.