Sidebar moving below page content

Hi,

The sidebars on my website were fine, but have recently moved below the content. I have done a lot of reading in the forums, deactivated plugins, and run it through https://validator.w3.org/ to no avail.
If I add another </div> near the bottom of the header.php the sidebar comes back up to where it should be, but shunts the whole page to the left (so it disappears of the screen to the left, with a heap of white space to the right). So I think this is the issue, I’m just not sure where this missing </div> tag actually needs to go to get this sorted.
Any help greatly appreciated.
You can view the website here

Also, I am using a child theme – and when I got the parent theme header.php and pasted it in instead it didn’t fix the sidebar issue, so it could be something else entirely…

Thanks