When we go outside the Euro-zone (Polish Zloty or Czech Koruna for instance), you get the best rate when you arrive in the country itself, but I'm not sure if that will hold for the Euro.
We're taking a trip to Bratislava (just for three nights) in a couple of weeks and they use the Euro, so is it better to get, say, £50-worth here and change the majority of our money once there, or get all the money here first?