Could always try putting a touch more torque on the head bolts*
Tighten in a cris-cross patten. Like you should do with wheel nuts.
*
will not be held responsible if a bolt shears off 
Pug 1.9 diesels are common for it. Lack of coolant changes is the main curlprit - old acidic coolant eats the gasket. Simple job, somewhat labour intensive as its typically French with everything being positioned in a ****-eyed manner. Not as bad as doing the same engine in a Pug Expert van....bad back anyone?
