Horizontal Line appearing above main menu

Hi wonder if someone can help me with this:

I’ve changed the Css of menu colours etc. in my Twentyfourteen theme – the main menu has a white background, but the problem is a thin horizontal grey line is appearing between the header (logo image) and the main menu. It is also slightly thicker on the right hand side of the page. Very strange.

Btw I am relatively new to WordPress, have created a child theme, but just can’t find the Css to remove this line, or anyone with a similar problem.

My test site is here: https://www.creativehosp.com/temp

If any one can help much appreciated!
Many thanks…