It's a good idea to check on the scheme rules. For example are there core hours you must work- maybe you are supposed to stay until 4.30, and certainly you must take a lunchbreak of more than 10 minutes!
And remember flextime is a two way street- the employer is pretty flexible about what time you can go but you need to be flexible and stay late when required to meet deadlines, provide cover etc. Also, check whether there is an early start/late stay rota to provide cover.