The minimum age for this licence is 16, and the candidate must demonstrate a laguage proficiency to at least level 4. A Microlight pilot is not required to have this licence to fly, however this licence is required to use a radio. Of course this is fundamental if planning to visit airfields.
There is a theory exam, which takes the form of multi-choice written exam, which needs to be taken first. There is a practical communications test, where a mock radio system is setup. The instructor will be in a different room, and will act as a Air Traffic Controller through a predetermined route, which is presented to the candidate. This proficiency test must be completed within 24 months of sitting the written exam.
When you have passed both parts of the exam, and relevant paperwork passed back to the CAA, the licence is valid for life.
Here is a LINK to the CAA page for more information.