Pic below is from a 2003 petrol Polo. It's taken from the side with the rear of the car to the left.
Am I ok to place axle stands on the big bit of metal highlighted by the red arrows, or the areas it is connected to? I'm guessing this is the sub frame? It seems quite solid.
You should be able to get the stands on the front of the big brackets that hold the rear axle beam to the car (top right of your photo) just in front of the bushes. Might have to unclip the handbrake cables.