This was just put up on the Madame Tussauds Facebook page. 50% off tickets until the end of April.
Usually the 50% is only for tickets after 5pm, but this looks like it is for all day.
I can't see this on their website, so looks like you can only get these if you click on the link, which is on their offical Facebook page.
My daughter has been bugging me to go and see Robert Pattinson, so this is ideal! Going this weekend