We propose to determine the magnetic structure of a new material, CeSbSe, at zero field and in magnetic fields up to 3 T, using the unique capabilities of the WISH diffractometer. We have recently reported our discovery of this new material. The special interest in this compound is that it may combine correlated 4f (Kondo) electronic state and magnetic order with topologically non-trivial band topology. A Dirac band crossing has also recently been observed in the related material CeSb. Our key finding has been that in a magnetic field, CeSbSe undergoes an unusual metamagnetic behaviour, where the magnetisation increases in fractional integer steps. This "Devil's staircase" can be explained in terms of the anisotropic nearest neighbour Ising (ANNNI) model. The implied frustration of magnetic exchange interactions promises an added potential for unusual emergent behaviour.