on cleaning my car today I've noticed some polystyrene hanging down behind the bumper, I've attached an image bellow.
I'm getting a bit annoyed with the dealers who didn't seem to check this car over properly but basically what I want to know is, is this important and do I need to worry about it?
I would say it's there for a reason and would be part of the integrity of the bumper. Even when you have a small bump it's best to get it checked for internal damage as if you have another then the bumper may not do it's job properly.
I would take it back and ask them to replace either the polystyrene or the bumper if needed. The bumper would need to come off, so maybe ask for photo evidence that it's been done correctly and they've not just removed what's hanging down. If in doubt ask vw or a body shop for an opinion.