The words 'Iran hostage crisis' were once enough to finish off an American president. The current one involving 15 Royal Navy marines and sailors isn't quite on the scale of the one that did for Jimmy Carter, but the stakes are still pretty high.
It's difficult to see how Britain and Iran can resolve the situation, without one or the other having to give in. The most obvious way to sort it sees either Britain admitting they were trespassing, or Iran bowing to all the international pressure and simply handing them back. Both very embarrassing.
And so it's become a question of saving face. The British diplomats are trying to come up with a way the Iranian leaders can hand the detainees back without looking weak and stupid in front of their own people. As this stunt probably has a lot to do with trying to boost their reputation at home as powerful men standing up to the west, Ahmadinejad and the rest must desperately want such a solution. But the mere fact the British seem to feel the need to allow Iran to save face at all, shows how strong Tehran has already become.