Thanks everybody for the information.
This article says that leaving the charger plugged in uses a matter of pennies a year, but that the real waste is people leaving their devices plugged in after the device has fully charged- this wastes enough energy to heat a bath every year.
I, and I suspect many others, tend to plug in the phone before bed until the following morning. It looks like it would save more energy by doing it for an hour or two before bed.
However, do devices stop drawing power when the device is fully charged and trigger the 'switch mode' that Kymmy mentioned? If so, the article is misleading.
http://www.techradar.com/news/phone-...night--1280918