Losing Coolant

I own an A2 and for months now, I keep getting the low coolant warning light. I check the coolant, and its low, so i top it up. This has happened for about 4 months now in total.

I have taken the car to the garage 4 times (twice to an Audi main dealer) and nobody can diagnose the problem. Someone has told me it might be the head gasket on its way, but no garage has been able to confirm that yet and there are no obvious symptoms that i can see.

Until recently, nobody could find where the coolant was going, but i've managed now to see that the coolant is being forced out of the coolant reservoir through the small release hole at the top. It seems that as i'm driving, the pressure building up inside the cooling system is enough to force the coolant out (is there supposed to be so much pressure?).

Audi have suggested that they need to strip the engine at a cost of about £400, just to see what the problem could be, so i'd rather not have to do that.

Has anyone had a similar problem with an A2 or any other Audi as i'm really worried that continuing to drive it will cause serious damage, but i dont have any other choice at the minute.