Page width becoming greater when scrolled to header

Hi guys,

I have a site over at shineboxoffice.co.uk which has an issue of scroll bars appearing when reaching the header of the page. I believe there was a patch for the twenty fourteen theme to fix this however I can’t update the theme for fear of all my other customisations going to pot.

Does anyone know of a fix as I have hunted and hunted around.

Thanks in advance,
Chris