I have had a huge coolant leak. It was slow but then eventually got to the point had to be topped up daily and didnt want to use Radweld.
anyways took it to my trusty mechanic and he had a look, said it was the radiator. Brought a new OEM radiator (TPS is next door to him was around £150ish I think) but he found that something attached (forgot what he called it) to the radiator behind the cross member, think like a aluminium bracket was lose and rubbing against the Rad. Caused a small crack. Checked oil cap and there was no mayo so that was definately a good sign.
He simply took it to a aluminium specialist and had it brazed to fix the Rad and put a rubber piece between whatever was rubbing against it. Cost £50 for the brazing. Anyways he checked thermostat, water pump etc and everything was perfect so said this was the best cheapest solution and the Rad is as good as new. cost me 3hours labour too (took him around 5hours so he did me a favour)
Anyways thought I would share my experience. £195 lighter in the pocket but its fixed with a permanent solution.
Thought I would share my experiences in case someone else has a similar issue.
PS my car is 2005 A3 2.0 TFSI sportback.