In the second and third embodiments, wherein at least part of the washcoat is in the porous structure, a size, e.g. a mean size, of the solid washcoat particles can be less than the mean pore size of the porous filter substrate for example in the range 0.1 to 20 ??m, such as 1 to 18 ??m, 1 to 16 ??m, 2 to 15 ??m or 3 to 12??m.