Useful documents include your medical records, appointment letters, test results, prescriptions, photographs, and a clear timeline of what happened and when. Keep copies of correspondence with the NHS, hospital, GP, private provider, or complaints service, together with evidence of financial losses such as payslips, invoices, travel costs, and care expenses. You can usually request relevant health records from the provider, subject to data protection rules. A solicitor regulated by the SRA can explain what evidence is needed and arrange independent medical opinions if your circumstances justify a claim.