Twntytwelve header image not showing

When I load an image at the recommended size (960×250)it adds to the Media Library but does not show the image in the header preview or on the site. The code to show the image does not appear to be triggered, so get_header_image() must be returning an empty string. But, I do have an image loaded and haven’t selected ‘Remove Header image’.
What’s occurring? my site is https://www.whitebears.co.uk