Sunfish,
the Iranian vessel was on its was to Syria in breach of EU sanctions and was therefore boarded. But Im sure you knew that already didnt you? The UK have also been in talks and offered a peaceful way forward.
It appears however, that Iran has proceeded in a tit for tat game. Itll be interesting to see if the UK vessel was in breach of any international rules/regs. The company who own it claim it hasn’t.