Why does Britain refuse to swap hostages?

In the last days of November, Dr Kylie Moore-Gilbert was released from Tehran’s Evin prison and flown back to a welcome in Australia. A dual Australian-UK national, she had served two years of a ten-year sentence for espionage — pronounced after a secret trial. She had been in Iran for a conference and was detained as she was about to leave. She had strongly protested her innocence.

So why, many Britons might ask, is the Australian-British academic suddenly free after two years in which the Iranian security services tried in vain to ‘turn’ her — while the UK-Iranian dual national, Nazanin Zaghari-Ratcliffe, remains under house arrest in Tehran, with the prospect of another trial and a return to prison. Employed by a media foundation in Oxford, Zaghari-Ratcliffe was detained during a visit to her parents four years ago, and — like Moore-Gilbert — rejects all suggestions of spying.