There have been a number of threads on the Polo GTI A/C issues before, so please do a search before starting a new one.
Other than that, I had the same issue as you when I bought mine...A/C gave out 3 days after I bought the car. Regas...Then they said they couldn't find the leak with the dye. Get them to have a look with a "Sniffer" in the drivers side foot well. Mine was found to have been leaking from a seal between the T/X Valve and the Evaporator. This could be your problem too.
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