What did Britain do to get the hostages back?
Speaking outside Downing Street, [PM Tony Blair] told reporters that the return of the personnel did not involve any “deal” or “negotiation”.
He expressed his belief that a combination of maintaining a “calm and firm” stance, which maintained bilateral communication while mobilising international opinion in Europe and at the UN, ensured the safe return of the sailors to Britain. — In the News (UK)
I think it’s safe to say that Mr. Blair was speaking in a somewhat technical sense. The capture of the 15 British personnel was obviously planned in advance, and not a response to an incursion (which did not happen), as the Iranians said. Therefore the operation was intended to achieve some goal, other than amusing Ahmadinijad. It’s reasonable to assume that something like negotiation occurred, and the quick release of the hostages implies that there was a quid pro quo.
So what do we think it was? The US is still holding five Iranians captured in Iraq; one possibility is that they will be released shortly. On Tuesday an unidentified Iraqi group released an Iranian diplomat, Jalal Sharafi (the Iranians claimed that the CIA was behind his kidnapping). This may have been the down payment. On Wednesday the US military announced that the International Red Cross had made a visit to the five in US custody.
One other, somewhat disturbing theory is that concessions were made regarding Iran’s nuclear program:
A senior E.U. official has told the London daily Al-Sharq Al-Awsat that a short time before Iran’s release of the 15 British servicemen, E.U. foreign policy chief Javier Solana spoke by phone with Iranian National Security Council chairman Ali Larijani on the possibility of negotiations over Iran’s nuclear program. — Al-Sharq Al-Awsat, London, April 6, 2007 — MEMRI