Update: Fire engines are arriving but are held back at the holding area outside the Central Bus Terminal.
Update: The terminal has just been reopened. Now hundreds of people are trying to get back in. It looks chaotic!
Update: I’ve now made my way to Kuala Lumpur but nearly missed my plane in London since I had to queue for about an hour for security at Terminal 3 and it seemed as most other passengers of that flight were already through it before the evacuation. Luckily for me the fantastic onboard service from Etihad Airlines made up for the Heathrow shambles.