Google custom search display problem

I've just installed this plugin and it's working really well.

It's just a display problem I'm trying to iron out. So far I've spoken with the theme developer and they've really helped with some CSS additions but we just can't get the search button to display properly. (I know I could turn it off but then the search bar doesn't display across the width of my search form)

As you'll see if you to http://www.animal-shelter.co.uk and click the search icon under the menu, the search button is cut off.

If I could just iron this out (or if I could replace it with a custom search button) so it displays correctly I'd be really grateful.

Thanks.