When I went back some kind lady told me that my car was in the middle of the road and that the car had rolled back - Doh!
The reason for this is that the handbrake works on the rear brakes - these are disc brakes.
When the pads and discs cool down, the handbrake may still be fairly tight, but the brakes loosen off and your GTI rolls back.
I will never be putting my POLO GTI on another hill without leaving it in gear