Below is a picture of how to use roundabouts.

This is the official picture used in DSA publications.

Turning Left - On approach check your centre and left mirrors then put your indicator on left 6-8 car lengths back from the roundabout, prepare the car to slow down and stop and I would suggest getting in 2nd gear on approach.  Stay to the left lane on approach.  You need to give way to any vehicles to the right hand side exit of the roundabout and any vehicles on the round about turning in front of you.  If the roundabout is clear then you can proceed to your exit.

If there is traffic on the roundabout then you need to stop and give way.  If you look at the roundabout from the perspective of the vehicle turning left you would need to give way to the right hand vehicle approaching the roundabout. Although if you look, the green vehicle turning right between the vehicle turning left and the vehicle approaching from the right, is preventing the approaching vehicle entering the roundabout.  This allows the vehicle turning left to progress without stopping.

Using vehicles leaving the roundabout to your exit of approach to block cars you should otherwise have to give way to is particularly helpful on busier roundabouts where there is a constant flow of traffic approaching from the right. 

By Alan Whittingham