I would make a new page type for your theme that is blank except for a single editable area where I would add a backstretch for the background.
For this I would use one of the Backstretch addons like:
http://www.concrete5.org/marketplace/addons/backstretch/...
or
http://www.concrete5.org/marketplace/addons/backgrounds-extended/...

You could have that page type call a different header or footer with different content to your usual header and footer. Simply create a new header.php file and call it something different like headerhome.php and the same for the footer and then you have a unique header and footer just for your home page.

There will be lots of other ways to do this but this was the first that sprung to mind, other may well have better methods.

Hope that helps

Many thanks for taking the time to post. I will work on getting my head around how to create a different page type that is blank.
The full screen background I should have little prob with.
I might have problem with creating a Title to appear over the fullscreen image background, without any white background but I'll deal with one hurdle at a time :-)

Cheers!

If you are not 100% on a blank page type you could always use the free theme Void.
You can have two themes active at once and assign your page to this theme.
http://www.concrete5.org/marketplace/themes/the-void/...

I don't think it is Responsive but if that's not a concern then this may be easier.

Thanks for the Void suggestion. Have looked at it ...not sure it's where I want to go initially. For one thing I want to put the same Autonav that is on the rest of my site overlaying this full page background. This might prove tricky. I might be better trying to work from the existing Marketplace theme I'm using. It also has an easy to implement full screen image.

I'm pondering!:-)