History has shown that a SAS mission to free the captured sailors would be highly risky with a high likelyhood of failure. They are being held in different places, and the fallout from a failed mission would be far worse than any grovelling. At the moment, a great deal of diplomatic pressure is being put on Iran. The EU countries and Turkey have been very supportive, though the UN response has been pretty poor again.
http://english.aljazeera.net/NR/exeres/0A0E908 F-E565-47AA-94A3-538F0B32476A.htm
I don't think the Americans would need asking twice for assistance. They have been spoiling for a fight since the 'Axis of Evil' comment by Bush. But I don't the US will be asked to intervene.
There are two different factions ruling in Iran. One will see the captives as an insurance policy - a human shield against an impending US attack, and the other will see it as a Diplomatic disaster further pushing Iran into isolation.
I think this could drag on for a number of weeks, with the Iranians taking every opportunity to humiliate Blair and the UK. Diplomatic pressure will see releases in dribs and drabs. I can also imagine a repeat of the Carter scenario where this will remain unresolved until Blair's last day as PM and then all the remaining captives will be released to the new incumbant at 10 Downing Street.
Then again, the whole crisis could have been engineered to legitimise a military attack...