I sometimes see people doing things I think are unsafe but I am not confident enough to challenge them myself. Should I just ignore it?

No.  If you do not feel confident, contact your local SHE Group.  Every member of the STFC has a responsibility to take care of their own safety and the safety of others – talk to the people concerned, their manager or the person responsible for their work in the case of contractors, or SHE Group, and remember to report it as a near miss incident.