A magnet assembly comprising a former having an outer surface (20) with a cavity (12) formed therein, and a coil (14) wound into the cavity (12), said cavity being of greater depth than the coil, wherein the coil is overlain with a layer of filler material (18) of sufficient thickness that its outer surface at least aligns with the outer surface of the former (20); a clamp (22, 30) is provided, over at least part of the surface of the layer of filler material, and fastened to the outer surface of the former; and the filler material and the coil are impregnated with a solid material.