A casing brush assembly (1) comprises a mandrel (2), upper and lower stabilisers (5,6), a brush (9) and a magnet assembly (10). The magnet assembly comprises a plurality of helically extending brush elements each of which is spaced from the underlying mandrel so that it can flex inwardly to accommodate variations in casing diameter. The thickness of the brush elements is relatively thin in the portions which connect the central regions to the ends of the brush with the result that the deflection of the brush elements is largely accommodated by the relatively thin regions. The magnet assembly (10) includes three rings of magnets held in a non-magnetic cage and located beneath a thin cover of non-magnetic material. The assembly can be used in a non-rotating mode (in which the brush is free to rotate relative to the mandrel) or in a rotating mode in which the brush is rotationally fast with the mandrel.