Hi there

Im hoping someone can help me fix this one. The car is a 2005 astra sport with a 1.7 diesel engine. I noticed recently the car was losing a lot of coolant... (1-200 ml on a 300 mile trip). Having checked the obvious (expansion tank, pipes etc) and for signs of a head gasket problem I noticed the source... On the right side of the engine block (on the top) I noticed a pipe (1cm diameter) with a L shaped top leading from here under the battery tray somewhere (cant follow). A small metal clip appears to be leaking the fluid form a black plastic joint. Could anyone tell me what this pipe is for?