disappearing logo until rollover

Hi, I have a problem with a child theme of twenty thirteen.

I've modified the header to display a logo but I'm getting a weird issue with it in Safari (8.0.4) on the Mac, every time I scroll down the page then back to the top, the logo disappears until I roll over the area where it should be.

http://achillea.aaronbennettdesigns.co.uk

It's not happening in Firefox or Chrome.

Hope you can help.