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).
This is a tricky one.

The basic answer is: You were there at the disposition of the employer in the time agreed upon with him; that you did not were able to do work due to factors external to you does not change that. It is not different to, say, staying an hour idle at work because of the computer do not work due to a power outage.

One small "but" is that it may be difficult to prove that you were there at 8 pm; but since you state you are there everyday that sould not be much of an issue. Also any message contacting your coworkers and reporting the issue with the doors should help. Check with your boss that he knowns that you were there at 8:00 am.

The big "but" is that you have just started with flex time, which means that the system still has to be worked out and your employer is still evaluating it. If the employer gets to the conclussion that the system leads to employees working less hours(either if it is not the employees'fault), he may want to return to strict time. I am not saying that it would happen by just one incident, but if it happens more times it could be determinant.