Flying to Sydney with layover in Beijing

Your itinerary is definitely and undeniably eligible for 24 hour visa free transit and you do not need any kind of visa. This is the most important detail. In other words, when you travel through China with a short layover, you are covered by 24 hour visa free transit that is counted from your scheduled arrival until your scheduled departure or better to say, it really does not matter if you stay over the midnight. Only the real hours are counted. By the way, you should know that UK citizens are eligible even for 144 hour visa free transit, so if you have a layover even longer than 24 hours and staying in one area like Beijing, you can enjoy 144 hour visa free transit. In any case, you are good to go...

Also, if you want, you can apply for a temporary entry permit upon your landing in Beijing at the transit counter. This application is free of charge and it allows you to explore the city during your layover. You should just pay attention to return back to the airport on time.