Question: a bit of background to the question, I'm a junior developer and we have recently been given flexitime which so far is working well as I get to start at 8am and leave at 4:30pm.

Today I arrived at 7:30 (I like to be early so I can have a coffee in the office before I start my work) however no one arrived to open the office until 9am.

So my question is, should I leave at the the normal time (5:30 pm) or when I would usually leave(4:30 pm)?

Answer: 
Unless there was a pre-existing conversation with your boss that you were coming in at 8 and that someone would be there to open the office then I'd say that this time you'll just have to take it on the chin and work till 5:30.

What I would do though is explain to your boss (in a non-confrontational, non-accusatory way) that there was no-one to open the office when you arrived and ask if you need to be letting people know about any plans to arrive "early" to ensure a keyholder opens up, or if the office *won't* be opening pre-9:00 on a day you were planning to come in early that you can at least adjust your plans.

Most likely this is just a blip that is down to the organisation adjusting to the flexitime (you mention it is a recent development).
With all implementations of flextime that I know, there is a set of rules, in particular **core and maximum work hours**. You need to be there at least during core hours (e.g. 10am-2pm), and you may only work during a certain window (e.g. 7am-7pm). Was that not communicated?

If not, then talk to your manager and ask them to set this policy. Then everyone will know what is or is not possible with flextime.

As to today: If you normally start at 8 and leave at 4:30, I don't see why you cannot leave at 4:30 today, too. The time from 7:30 to 8 is probably on you, because it was not clear whether you can start before 8, but if your normal day starts at 8 am, and the door is locked, that's not your fault.