Are you saying it is still loosing coolant out the header tank? OR that the coolant level in the header tank keeps going down?Well, have had head gasket replaced...consumption is much worse now!! Is it normal after head gasket replacement?? What cause this?
If loosing coolant out the header tank it should be visibly 'damp' around that area.
As mentioned previously by Brian Gordon-Stables - my money would have been on your EGR cooler - this is just a heat exchanger between your exhaust gasses and coolant - it this fails you will see exhaust emissions in the coolant and it will over pressurise your coolant meaning it will keep loosing out past the header cap.
The latest VAG part number is 04L131512BQ which should be around £200 for the part.