According to at least one aspect, the invention provides a filter assembly (100) comprising a tubular housing and a filter device (70) locatable to extend longitudinally within the housing. The filter device (70) comprises a lattice structure of crossing internal and external elongated elements (18, 20). The lattice structure comprises an arrangement of pores (22), and the external elongated elements (20) are presented outwardly of the filter device (70) with respect to the plane of the pores, thereby to provide ribs on the exterior of the lattice which define a shaped channel that will act as a first level filter for larger particles. The external elongated elements are shaped and arranged to provide one or more additional filter levels for smaller particles in advance of said pores, through which yet even smaller particles may pass.