So,

Operative leaves site stating they've hurt their back, advised the supervisor, instructed to report to H & S for Accident book etc

Doesn't report to H & S, emails in their account of events two days later with names of co-workers with him at the time.

Investigation discovers that the 'IP's' account of events, including presence of co-workers at the location and time of the event are different to the witness statements of the event, who weren't even at the location at that time.

This will likely run over a 7-Day absence, so is it RIDDOR or a potential Disciplinary

It's not either/or - it is perfectly possible for an incident to lead to both a RIDDOR report and discplinary action.

Lying about the circumstances of an incident does not automatically disqualify it from being RIDDOR reportable. It is only on the facts that you can decide whether it is reportable.

I agree with Chris, a bad back could just be that and not an actual reportable event, i would follow process as normal for the injury and treat the conduct investigation as a seperate entity for now, i'd carry out a resasonble investigationg before assuming an employee is lying or exaggerating an injury. 

Pete, to some extent I am inclined to play Devil's Advocate here!

"Operative leaves site stating they've hurt their back, advised the supervisor, instructed to report to H & S for Accident book etc

Doesn't report to H & S, emails in their account of events two days later with names of co-workers with him at the time."

What does your organisation's policies say about who reports to the dreaded H&S?

....and if it is down to the injured person (however injured and to what severity of injury) does the policy set down a timescale. Do you actually expect someone in hospital with a broken leg (as example) to report to H&S before getting in the ambulance or would an email two days later be fine?

Isn't it the role of the supervisor to elevate the information given to H&S?

"Investigation discovers that the 'IP's' account of events, including presence of co-workers at the location and time of the event are different to the witness statements of the event, who weren't even at the location at that time."

This smells a tad fishy! 

Why would the IP make up their understanding of who was around when their reported accident occurred, if an investigation is going to conclude that a different narrative is going to appear?

The reported incident and thence injury is unlikely to be dependent on needing some witnesses to identify in the IP's email.

Now I can think of loads of reasons why you might have this discrepancy. As example may be the witnesses SHOULD have been around but were not..The IP COULD have been trying to cover for them, not realising that they would come clean when asked!!

You alone are never likely to have enough information to conclude that this absence resulted from a pre-existing injury.

Personally I would go with the route of trusting what the IP says until you have solid evidence to the contrary.

What message does it give the workforce if you start from a position of doubting what one of your workers (complete with injury) tells you? - will the next victim simply not bother? [LOTS of history of similar happening across the Globe!!]

Is one more on your number of RIDDORs a game changer?

You report. Is HSE or other enforcing authority going to investigate? On the strong balance of probabilities NO.

You have the opportunity to consider whether there is room for improvement. May be there is, may be not. But why waste the opportunity by trying to deny what is reported?