Astra J Internal coolant leak

[benawhile]

My 2015 Astra J has started losing coolant. I can't see any external leak. I understand the possible causes are
Blown Head Gasket or even warped head.
Leaking Oil Cooler.
Corrosion at Inlet Manifold caused by issues with Dex type coolant.

Can anyone say how to determine which of the above just to check I am in agreement with my mechanic before I contact him?

[stevemcs]

Have you pressure tested it ? THe gasket on the EGR cooler fails on these too well on certain engines it does.

[benawhile]

Thank you Steve. I will ask mechanic about pressure testing.
About the EGR. Can water leak externally from there or can it also leak internally into the oil?
I am losing about 300 ml in 300 miles so I think it should be visible if it was external

[Huzzah]

If its small amounts onto a hot surface, it evaporates straight away leaving few tale tail signs.