It seems related to boost control and is currently causing my dads Limp mode as there is a loose pipe that seem to have been connected onto it at some point and now no longer is.
its currently blanked off with a screw and he is taking it easy so as not to overboost the engine but i just cant seem to find it in anything logical on Service Citroen
Update, managed to work around the new flickr UI and get the direct image link!