To reply to myself
Maybe I have just forgotten how to do this, but I have an existing
file that I used the article (koma+ beamer) class with OR article
(beamer) class .
However, switching classes just compiles as an article and not a
presentation.
Switching to the Beamer class gives me a presentation but with slides
that are two to three times the depth they should be. But otherwise OK.
Just in case it was a Lyx 1.5.6/Mactex 2008 thing. I updated to lyx
1.6 and its exactly the same.
However, the sample Beamer file works fine .
It was of course me getting confused, The slides were the wrong size
because I had custom page margins, which I assume were needed before,
but not needed now.
Also I am amobviously confused about the differeces between
article(koma+beamer) and just article (beamer)
