Hi Ed, the one i found that i think will work for me is at
premium.wpmudev.org/project/the-pop-over-plugin/
There you can find more details. I became a premium member a while ago so for me it was just a download, but i am not sure what the cost would be otherwise.
The plugin allows for several rules to be created to dictate when it will be shown. The one rule that appealed the most for me was to only show on the home page, and only when the user is not logged in. Other plugins failed to allow which URL to show the 'splash page' on, so that meant where ever you are on the site the page would show for example. Another rule important for me was that it won't show if you are logged in ( registered). Ordinarily i don't design with these types of interference, however a client wanted it... and he pays the bills...
The other nice thing about this plugin is that it is a normal wp page, so you can pretty much embed a video, raw html, use widgets in your theme on the page etc.
thanks again for your support.