Page redirect problem

Hi all.

The redirect problem I have is on this URL: http://www.plotkingonlineprinting.co.uk/order-prints, where it is meant to go an order page but it redirects to this page for some reason: http://www.plotkingonlineprinting.co.uk/wp-login.php?redirect_to=http%3A%2F%2Fwww.plotkingonlineprinting.co.uk%2Forder-prints%2F, and it only redirects to this when I am not logged into the admin area. Once I am logged in, it goes the correct URL. Funny one I know.

Any ideas anyone?