Varley, there are only two things that can cause the suspension to sit lower on one side. As I'm sure you know the rear suspension is very simple on the Polo. It can only be either the springs which holds the weight of the car or the beam itself.
You say you have checked the spring so we can assume it hasn't broken, which is fairly common. I doubt it has a badly sagging spring on just one side either so I don't reckon that's the problem.
IMO much more likely is a bent rear beam. Does the car apppear to have more/less camber on one side? To check the beam isn't twisted, ideally you'll need to take it off and lay it on a flat surface. Does it lay completely flat? Take some diagonal measurements to ensure it's "square" etc.
The beam bushes could possibly be badly worn, though I've never seen a set that are completely shagged out - even on a 170k heavily abused example they were ok.
The rear beam itself pivots off brackets that bolt to the body. The bolt holes are big enough to allow some movement which can be used to help align the axle laterally and fore and aft. This wouldn't cause the car to sit lobsided though.
Hope this helps,
Richard.
