I've built a high-poly model and manually reduced it to low-poly, by applying the subsurface modifier and merging excess edge loops. I then imported copies of these in .obj format to XNormal, where I rendered a normal map. When applied to the low-poly model in Blender, it recieves these fine, rippling artifacts:
Comment on answer by cmomoney: Thank you, this was exactly the problem. I applied the Mirror mod and remapped the UV. Now I've got problems with the high-poly poking through, so I'll have to remake the low-poly anyway... but that is beyond this question.