Sticky LEFT sidebar....

Some weirdness here.
I simply want a stocky left sidebar on a page that has a header, menu, left sidebar with content area, and a footer.

I have seen the code - here -for a RIGHT sticky sidebar VIZ

@media (min-width: 769px) {
   #right-sidebar {
		
        position: -webkit-sticky;
        position: sticky;
        top: 0;	 		
    }
}

and thought - optimistically that changing #right-sidebar to #left-sidebar would work...
Sadly not! I DO end up with the left sidebar sticky ... on the right side!

For avoidance of doubt, this is what I have in the css:

@media (min-width: 769px) {
   #left-sidebar {
		
        position: -webkit-sticky;
        position: sticky;
        top: 0;
		 	 
		 		
    }
} 

The site is a test site with GP pro, all modules except woo active and only the gp and simple css plug in... The left sidebar has a single text widget in it.
I have NO idea what's going on. The problem is consistent across a live site (so I am not using it there).

Help?

NB I tried adding a

left: 0;

But that had no effect at all..

PPS I KNOW I can use a plug in to achieve this - and I have on a live site but - really - can't see why I can't do it with simple CSS.... :(