Contractor Among Contractors
It will depended on what they do for you and what you do yourself already.
Payroll year to date - record of deductions, copy of all RTI submissions, and all,other payroll records held.
If they do your bookkeeping, then you'll need trial balance and nominal,ledger year to date, plus return of prime records.
Copies of all vat returns and vat workings.
As a matter of law, all papers relating to tax compliance are your property. Likewise basic accounting records are, pursuant to the Companies Act, your property. The accountants own schedules and correspondence are theirs.