The critical properties of the modulated phases in the uniaxial chiral clock model in three dimensions are investigated by a renormalisation group (RG) calculation and a Monte Carlo study. From the RG calculation it follows that the model belongs to the XY universality class, i.e. that it scales isotropically at criticality in spite of its spatial anisotropy. Using the scaling properties of the fourth cumulant of the order parameter, the authors devise a method by which one can discriminate between isotropic and anisotropic scaling quite generally in a numerical simulation. The numerical results confirm the predictions of the RG calculation.