A Royal Navy ship reportedly aimed its guns at Iranian boats after they attempted to seize a British oil tanker.   HMS Montrose was escorting the British Heritage tanker through the Persian Gulf when the stand-off with five Islamic Revolutionary Guard Corps boats took place on Wednesday, according to CNN.  It is claimed the boats told the tanker to change course and enter Iran's territorial waters, with a US aircraft recording the incident.  HMS Montrose, which was trailing the tanker, then trained its guns on the Iranian vessels and warned them to back off.  The Iranian boats obeyed the frigate's instructions, which were issued over the radio.  Thirty British commandos stormed an Iranian oil tanker bound for Syria last week, prompting Iran's President to threaten the UK with retaliatory action.  Britain's Ministry of Defence had no immediate comment.   Iran was accused of attacks on six tankers either in or near the Strait of Hormuz, which connects the Oman and Persian gulfs, in the last few months.  The US and Saudi Arabia, its major ally in the Middle East, blamed Iran for the incidents, although Tehran denied responsibility.