If it ever goes to court, even in a civil case for damages, they will have to disclose everything. If the contractor is seriously worried they can put in a formal written request to the Health & Safety Manager or a Director. They may well be unaware of the situation however I suspect a cut is not worth the effort.

However there may well be vested interests manipulating and directing the investigation to either save face or avoid any criticism of their own actions. Unfortunately where this practise still goes the poor contractor can finds it impossible to do anything.

An all too common affair - sad to say. First, it's unlikely you can carry out a proper investigation without the assistance from the contractor i.e. injured party. Generally there is no legal duty to carry out an investigation so you do not have that to fall back on. That said, and assuming there is a CPP for the project written by the PC you may be able to extract something useful which may state...'all accidents with be thoroughly investigated' blah, blah. I would be tempted shame them by reminding them what they have written adding that a CPP is also a document required by law which can be used in evidence - or something to that effect.

Has the client any hands-on involvement or are they one of those inert entities?