Inside the Georgian mansion in Mayfair that houses Saudi Arabia’s embassy in London, two offices of Saudi intelligence operatives are busy at work. 

The first office belongs to the General Intelligence Presidency, Saudi Arabia’s foreign intelligence agency. Its staff are engaged in the “normal” work of spies, meeting with MI6 to discuss Britain and Saudi Arabia’s shared fight against al-Qaeda or to swap secrets on Iran. 

The second office has a darker purpose. There the staff of the Mabahith, Saudi Arabia’s secret police, are tracking people of interest inside the UK. 

Its operatives are watching obvious targets like Saudi dissidents who have gone into exile in Britain, or London-based Islamists…