He told me that underneath the headertank there's a brown canister type thing with a few pipes coming out of it which is the culprit.
He says he's going to strip it down in a while to have a look exactly what the problem is, but ATM the car's not usuable...
A quick look on Service Citroen for him brings up this, and I suppose the circled part is the part?

Anyone know what it's called/partnumber, as it doesn't seem to be on there?
Thanks!