I have a jaguar s type 2003 model 2.5ltre which constantly keeps sounding when at rest like a kettle boiling water. i have had it serviced by a jaguar main dealer 6months ago and had to return it back on 6 occasions with the same problem. i am still hearing the noise which i now am discovering that an oil is leaking comming from behind the top end of the engine rocker cover dripping onto the heat manifold sheild, any ideas where this can be traced from

Valve covers (or Rocker covers) are notorious for oil leaks on many types of cars. (Dont know yours) BUT If it is hot enough to boil oil, you have a serious FIRE Hazard ! That could easily burst into flame any time.

Are you sure its not water seeping out of the head gasket? That would readily sizzle on the heat shield and constantly sound like boiling water. Makes more sense to me.

What type of Jag dealer would miss this? Six times ! I would try a reputable local mechanic.
good luck