A big part of the problem is that the Iranians aren't speaking with one voice. IIRC, the Foreign Ministry said the woman sailor was going to be released. The next day another Iranian group said she wasn't.
There's no doubt that it's a complicated situation. My guess is the Ayatollah or his deputy will have to tell the groups what to do, and after that it'll be resolved fairly quickly.
All bets are off though if Iran puts on a show trial or the like. I don't expect the UK to take military action, but things could get very messy.
We'll see.
Cheers,
Scott.