Replacing fossil fuel burning synchronous generation with asynchronous renewable generation to deliver environmental goals will significantly reduce system inertia. Reduced inertia allows a faster and larger frequency deviation after a disturbance and the reduction in inertia in GB will be significant enough in the next decade that the existing frequency control will be too slow to contain the frequency deviation after a large disturbance. Therefore, delivering fast, coordinated frequency control from new service providers, e.g. energy storage, (termed 'smart frequency control') will be vital to overcoming the challenge posed by reduced/variable inertia in GB. This paper describes some of the challenges that must be overcome when delivering this form of control, in terms of controller design and the definition of a new ancillary service, alongside simulation results for a 36 zone equivalent model of the GB frequency response that illustrate these challenges and the threats posed by reduced inertia.