My car has a leaking radiator with overflowing reservoir

Hello, I have Hyundai i20 CRDi diesel. My car has leaking radiator with overflowing reservoir. Hyundai Service center has diagnosed overflowing due to head gasket failure, they has changed the head gasket, valve seals and the installed the new radiator. They had not changed the thermostat.
But after the repair I drove it for 600km and again the reservoir started overflowing. They had repeated the job by re-surfacing the head, and fitted the components with previously used head gasket.

Now, again after driving around 1000k coolant getting low in reservoir. As per there suggestion that coolant take time to adjust fully in the system, I had added the coolant twice in reservoir and topped up the radiator cap. Again the problem is still there coolant getting low in reservoir. There is no signs of coolant leakage underneath the car, they had also inspected the hoses and bolts for leakage. NO signs of coolant mixing in oil or no white smoke. They do not have any machines for system testing
Now they are asking me to drive it for at least 3000km, but i'm afraid because they are giving me the warranty for 10000km and in between I had driven it after repair for almost 2500km while visiting the service center. MOre 3k will worsen the situation coz they mostly deny to repair under warranty.

They are trying to ignore me with the same problem and foreman's face shows that he do not have the solution for this problem and they are trying to pass the time till getting 10000km that are under warranty.

plz help to fix this issue