'correct interpretation of exactly what an employers duty is in this scenario' Good luck with that! I would say no unless you can confirm that the sickness is a direct result of the original accident. If its a case that they cut themselves and then get an infection a few days later because they didn't keep it clean or something along those lines, I would say no.

I wouldn't report it.

I second what pl53 says as we had a similar incident last year and I wasn't sure so contacted HSE for advice. Our scenario was and employ cut his finger and attended local GP surgery for initial treatment and returned to work the same day. After attending the GP surgery a few days later to have wound checked/dressings checked it became clear that the nurse hadn't cleaned the wound properly (to which she admitted she had concerns) and it had become severly infected which resulted in surgery under general aneasthetic to clean the wound (it was so bad there was a risk our employee would lose their finger but thankfully that didn't happen). Anyway employee was then absent for more than 7 days. HSE informed us that the fact that they had returned to work following the initial incident (cut to finger) broke the chain of events between the initial work place accident and the subsequent infection and therefore it was not reportable under RIDDOR.