Steve,
If you advised of the potential risks in theory you have completed your duty of care as far as H&S goes this leaves the company at risk of any litigation for any secondary medical issues their staff are affected by. Though I would drop them a line and keep a copy on file should you get a visit from HSE advising your preferred action & discussion . Did you advise them regards fecal coliform, pathogens, etc etc. We actually develop dip slides & develop these in an mini lab then take photos and attach to the file for each sewage job before & after treatment. The health risks are too high, strep, typhoid, dysentry, not to mention good old Hepatitis A. I hope you sanatised your machines well.
Cheers
Jim