Helen, all you have to remember is this. Pilots do a hell of a lot of training, both in simulators, and in the right hand seat of the flight deck, long before they become even a Co-Pilot. The they are in the right hand seat for a long time before qualifying even further are going over to the Left hand seat, the Captian himself.
The two or three guys on the flightdeck know what they are doing, with everyones safety in mind first. If they have a reason for doing something as described above, then they justify that reason, and then take action, and not before.
In the instance of fuel, most aircraft flying from the UK, UK owned aircraft will normally have enough fuel to do what they want. Rather than keep flying round an airport using more fuel on attempts to land or circle, they will fly back up to cruising height, using less fuel once there to go else where until the problem is resolved.
It has been said time and time again, flying is one of the safest forms of transport. Just sit back, relax, and enjoy it. Happy flying.
Yeia mas, Chris.