oil in the water top up tank astra 1.7tdi?

iv'e got oil somehow in the header tank,top up for the radiator.
there is no signs of water on the dip stick or oil filler cap,also after running the engine for 1 min the header tank does not hiss.all signs of head gasket trouble "i think" so why have i got oil in my header tank??????
any help appreciated
thank you