Attending the Old Bailey as a spectator

If you want to go and sit in the public gallery of the Old Bailey court room to watch a trial, how early before a trial should you rock up, and what do you say to and to who when you enter the building? I’m sure it’s not “Hi, I’m here to watch a trial. Do you have any in session I can attend today?”

Sorry for what might seem a dumb question to many, it’s just something I’ve always wanted to do and never have.