Hi Darren
First of all you need to enable DOF in the Scene Tab in Render Settings. Then you need to define the f-number of your cam, I usually do this by adding a CoronaCameraMod to my Standard Camera and set the f-number there. Depending on the focal length of your camera, the DOF will appear greater or shallower in your render. If you have a similar focal length like in your attached image, then for getting a strong DOF effect you need to go down to something like 1.4 or 2.8. Generally it's easier to use long focal lengths to get emphasized DOF.
I hope that helps.
Regards,
Ben