My brother has a 2011 Golf TDI and it appears to be losing all of its coolant quite quickly.

Are there any common causes for coolant leaks and after having a quick look, has anyone tried this stuff?;

http://www.amazon.co.uk/K-Seal-Permanent-Coolant-Leak-Repair/dp/B002X2S3PW

Wasn't sure if this could save some money at the garage?

Has the head gasket gone, good cause of fast coolant loss i would keep well away from products like that.

The common cause is Hoses & Clips causing the leak,

so you look at that long before you get to jumping to the idea of head gaskets and water pumps or bigger failures.

 

But the water pump is a common reason, 

so best look at diagnosing the faults, simple to fix, needs some skill, or is going to cost you big, and do not just bodge the fix.

I'd be very surprised if the head gasket has gone! Hoping it's just something more simple.