Nothing in law to say do so but common sense dictates you make a note for that time two years and 364 days hence when a solicitors letter appears claiming "negligence and breach of statutory duty"

Thanks, that's what I thought but the main contractor didn't seem to think it was neccessary.

If an accident happens at work, it may or may not be ‘work related’ . if there is any doubt it would be prudent to investigate it and establish whether it is work related and might give rise to a claim or not. When people make a claim the first thing that the claims company asked for is the incident report.  If that concludes that yes there was an incident and someone was injured but it was not genuinely work related the employer is in a better position than trying to go back months or even years to establish what happened, and having to ‘prove’ that this was not a work related injury.