Often the first hurdle when a poor reference is suspected is to get hold of a copy. A reference includes 'personal data', which is protected by data protection laws, specifically the General Data Protection Regulation. Any request for a copy of your reference should be directed at the recipient of the reference, not the writer. This is because data protection law contains a special exemption for the writer of a confidential work reference (although a reasonable employer may volunteer a copy, especially if it is largely factual).
The position is different with references received by a prospective employer from someone else (typically your ex-employer). Here, normal data protection principles apply. Your request for a copy may be met with the response that handing over the reference would breach a genuine duty of confidentiality owed to someone else – for example a manager named in the reference.
The content of the reference would have to be genuinely confidential for this to apply, in which case, the organisation that was sent the reference would have to ask the referee for their consent before handing it over to you. It may not be confidential if, for example, you already knew the name of the person who gave the negative reference. Even if it is confidential, it may still be possible to supply the reference with the manager’s name and other confidential details deleted, or a summary of what it says.
The Information Commissioner says it is good practice to hand a reference over, or at least a substantial part of it, especially if the reference has had a negative impact on the person concerned, for example stopping them taking up a provisional job offer.