Hi Clive,
YouTube should just work these days, since it falls back to using a HTML5
player. If it's not doing
that automatically then you can go to the following page to check:
https://www.youtube.com/html5?gl=GB
I think you may need some extra gstreamer plugins installed if it show you don't
have full support
for the required formats.
I wouldn't bother with the Gnash or Lightspark plugins that attempted to create
open source
equivalents of the the Flash plugin. I found them very buggy and incomplete even
when they were
under active development.