Unwanted horizontal scroll bar appears

An unwanted horizontal scroll bar appears and part of the header is going off the page even when dynamically resizing. This goes away when the site is resized to a very large window. But at normal window sizes it is affecting the the site negatively.

Can anyone please suggest why this is? I have reduced the margins to 0 so the content will fill the screen but I don't want it going off the screen.

Thanks, for your help. 

You try adding this code to Website Tools (under Not Linked) > Custom CSS

body {
	overflow-x: hidden;
}