Everything worked out perfectly! Costas was bery spontaneous and accepted our request for the next day right away. The flat is cosy with a big room and high ceilings. It is located directly at the U1 subway which takes you straight to the city center, close to all the Christmas markets, Naschmarkt, the Prater, the Donau insel, etc. Also it is a short walk away from the main train station. Great place to stay!