The European Spallation Source is a linac-based facility that will be the world's most powerful spallation source. However, there continues to be a case for a new synchrotron-based spallation source facility in Europe that provides neutron pulses on a short (~microsecond) timescale. A design for such a facility is under study at the ISIS Neutron and Muon Source at the Rutherford Appleton Laboratory. Alongside an RCS option, a fixed field accelerator option is under investigation. Such a machine offers significant advantages in terms of the energy efficiency of the magnets and in the flexibility of operation. A novel design for a fixed field alternating gradient ring that maintains fixed tune across the momentum range while minimising the machine radius is presented. In addition, a scheme to directly inject protons into the ring, avoiding the losses associated with charge-exchange injection, is described. Finally, the design for a prototype ring, using the Front End Test Stand as an injector, is discussed.