Is this an unreasonable request

I've taken on a new sole trader client and have received some of the information I requested last year from the previous accountants.   The latter seem, however, reluctant to let me have their HP calculation on the client's van.   They've sent the original HP agreement only.   From this, I can't fathom their calculation of the HP interest and balance forward in the last accounts.   They seem reluctant to send the calculation.   Is my request unreasonable?.   What's to be done as time is of the essence.

Thank you.