There's only one thing you can do if you REALLY want something back! And that's violence.
I know you're more than likely have already thought about that and you're looking for other options but I don't think there are any.
If you're willing to let €400 go down the shitter, then walk away from it and put it down to experience but if you're like most of us who haven't got that much money to have robbed off you then you will have to take action and the options aren't many.
I hope you're able to get dough back Gem, you seem like a decent sort of bird from what I've gathered on here, so