What's an RSL, Mary?
Briefly, for those unfamiliar with UK radio, anyone can apply to Ofcom (formerly the Radio Authority) for a Restricted Service Licence (RSL). The RSL allows the licensee to broadcast for up to 28 days, either on FM or AM, with a very limited power output.
The main object of these short transmissions is for those planning to apply for a full-time licence, or community radio licence, in a particular area to 'test the water', but often stations are set up simply to cover special local activities, such as carnivals, air shows and sporting events. There have been successful RSLs to recreate the sound of Radio London, and Radio Caroline has also organised a number of RSL broadcasts.
Most applicants are granted licences, no matter what their standard of broadcasting (or lack of), but they have to specify exactly what they intend to do for the duration. Deviation from this is not permitted. If you tell Ofcom you are going to play music from 1967, you can't suddenly decide to include a current chart show in your schedule. (Not that you'd want to, of course...) For full details of how to apply for an RSL licence and the cost, visit the Ofcom Website.