In a tokamak, misalignments of field coils and coil connections, introduced in the construction of the device, lead to small asymmetries in magnetic field known as error fields. Even at small levels these can stop the plasma rotating, driving resistive tearing modes that lead to plasma termination. Recently, a set of four error field correction coils has been installed on the MAST spherical tokamak. These coils have been used to correct for the error field, allowing MAST to operate in previously inaccessible regimes. With error field correction, MAST can operate at densities at least 30% lower than previously attained. The error field coils have also been used to apply large error fields to the plasma to explore the phenomenon of mode locking. It was found that the locked mode threshold scales with line averaged electron density, safety factor and toroidal field in a similar way to that found from conventional aspect ratio tokamaks. This scaling law has been used to estimate the locked mode threshold for a proposed power plant and a component test facility based on the spherical tokamak concept.