Found the explanation. It selects a page at a time.
Also found that Acrobat Reader (version 7) will do at least do the first part of what I want to do so at least Adobe thought this was worth doing..
Under Edit->Preferences there is a category "Full Screen"
When you select that, there are various options you can choose in the Full Screen Navigation frame:
Advance every __ seconds [you to fill in __ ]
Loop after last page
Before I have a go at coding this thing does anyone know of something that works for Windows generally?