Neil, this sounds like a job for an Occupational Hygienist with specialist knowledge in this area, along with your in-house chemists who should know the properties of the substances present and being created. Contact BOHS at http://www.bohs.org/aboutus/contactus/ If your existing COSHH assessments have "holes" relating to nursing mothers it is feasible that other exposures are not adequately assessed or controlled. Your returned employee should be an integral part of the assessment team as she knows what the job entails, the way it is really carried out (ie not always what the procedures say) and has concerns over the her health and that of her child.