To be honest. Wouldn't be more fair-play to call the nurse that haven't signed the drug chart? Or ask a patient if he received an insulin (if he is not confused of course)? Writing an incident report form will not help you. IRF won't tell you if patient had it or not.
I have worked a bit in UK and came to conclusion that there was no solidarity between most of nurses. We are human, not robots and we have a right to make mistakes, especially working under pressure and constant stress. If you feel like first right thing that need to be completed is incident report form then do it. But do not be suprised afterwards when your colleauges will talk behind your back. I have never met a perfect nurse. Take care.