This has been discussed from the point of view of an employee requesting a copy of the accident book and there were similarly views for and against.

You do have the great cop out bandied by many firms that you either need to heavily redact the document so as to protect the personal information of others or it is simply unavailable.

Personally I would take the stance that the "Accident Book" and its content are a matter of company record in completing legal duties as an employer - they are not mandated as a publicly available record (hence the current official publication containing tear out pages).

If this is merely a general member of the public (and not the injured party or their legal representative) you should stick to the GDPR approach as you have no way of discerning a link between the two parties - lawyers must carry indemnity insurance the general public do not.

Assuming that the person is member of the public who has been involved in an incident on your site, I would simply ask them why they require this information.  I would tell then that the information is collected for the purpose of an internal investigation and if necessary to comply with your statutory requirement to pass on details of certain incidents to the HSE under RIDDOR. When the make a request they must establish what they intend to do with the information as it might contain confidential information that you simply cannot share with anybody. If they are making a claim then they should ask for the information through their legal representative.