sounds like the usual warped or cracked  plastic cover. just had the same problem on a 2.2 dci espace which is the same head ,block.tried new gaskets first . physicly inspected first,could not see anything ,but they are quite a complex moulding.easy enough to change , but the injectors have to come out,and also a pricey piece of plastic .fit a new gasket set also or be prepaired to do the job twice.
                 jamie
                
 January 2012