(31 Jul 2015, 6:18 pm)GX03 SVC wrote go to the management centre, past the metro towards the car park (past the pub and Costa). Ring the buzzer at the door, the person at the desk will ask you what you want to do, "take photos of the trains", and then they'll ask for your name (this is just for an evacuation, so they can check all the visitors are present). They'll issue you with a ticket which you show to the person at the gates, once you've finished you return the ticket to the office.
The last time I was there for photos (for Tornado) was when I was on a Get Around (so no rail tickets/Explorer to get me through with the assumption I was going to travel), I was with Adrian and Adam. We didn't go through any of that procedure, we asked at a little information kiosk, and got redirected over to the ticket barriers where we asked for permission to go through. No questions were asked.