For the most effective and efficient distribution of heat, BestHeating would recommend leaving a clearance of 50mm above, and 100mm below your radiators.
By creating clearances of these sizes at either end of your radiators, you can guarantee the warmth emitted from them will be able to travel around the room freely.
Our Advice Centre blog, Where to Position a Radiator, offers more detailed radiator positioning advice, should you require it.