"I think we can be pretty sure that if we had managed to crack a large terrorist ring the powers that be would be shouting it from the roof tops."
But you forget the adage: don't let the enemy cotton to what you're doing. Even if you crack the ring, there will always be stragglers and wild cards still on the loose. If you stay mum, you can mole them and keep finding conspirators. Usually, when the news releases word of cracking a ring, it's something shameful like drug dealing or child prostitution; but if it's something genuinely dangerous with positive motivation (like starting a revolution), they'll keep quiet to avoid a "no such thing as bad publicity" scenario.