Sylvia communis. This is to be found in the superb Bohemain aviary: a large walk-through, with some smaller, enclosed aviaries within. A large number of unfamiliar (in zoos) passerines are on display - according to Zootierliste, these are the only whitethroats in Europe, for example.
You might very well be correct - they're quite hard to differentiate (or, at least, they are for me!). Either way - unusual in captivity (although the redstart is slightly less unusual).