Coolant loss!

[quest63]

I have an ongoing coolant loss problem.

My 1.9 tdi estate started to lose the volume of its header tank in around 10-14 days say maybe 150-200 miles.

It went into a VAG independent garage who changed the thermostat housing as there was evidence of leakage in that area.

However the loss continued at what seemed to be exactly the same rate unabated.

They had it back again and had it on a pressure test for an hour with no loss of pressure and I was advised it perhaps had had an airlock to explain the drop in coolant.

Big airlock imo so I wasn't surprised when the dash alarm came back on to tell me the header tank was empty again in the same time / mileage frame.

So basically I'm looking for ideas and a solution. It occurs to me that even if the pressure test tests the whole system it isn't testing the header tank cap could this be the problem? Also I noticed a trace of coolant a couple of times on what looks like a spigot / blank on the underside of the header tank fairly directly below the filler cap is it possible a pressure test would miss a leak there?

Any help gratefully recieved

Thank You