Thanks Mary and Dave,
After posting I had a look around other forums and as you suggested, this is a genital problem, probably related to Internet Explorer. I have converted them to .html multipage files; surprisingly it seems to work very well. They open with a navigation bar on LHS with slide titles, as with PowerPoint files, but if you click on "Slideshow" in bottom RH of status bar, a full screen slideshow opens in a new window.
Thanks again.
Paul