Twenty Thirteen menu not visible on mobile

Just been look at our website (http://www.voicesforthelibrary.org.uk/) on mobile (iOS) and when you click on the menu box at the top it drops down as if a menu is there, but the text is not visible (if you randomly touch below the menu box it is clear the hyperlinks are there and active).

Any ideas what is causing this and how to remedy it? It is the basic TwentyThirteen theme without modifications (we’ve not fiddled with the coding etc).

Thanks.