But as secrets go, it certainly isn't a very well-kept one. In September 2013 the Guardian reported:
"Pizza Express uses halal approved chicken."
The same month the Independent reported:
"Restaurant chains that are changing their menus include Pizza Express, which uses only halal chicken."
But it's not just other media who beat the Sun to this exclusive, an FAQ section on Pizza Express's website states:
And they've been telling people this on Twitter since 2012:
So not that secret then.