To me that would depend from who the letter is coming from. If it is coming from your dad then i would let them know they are no longer needed via a disengagement letter.
If you are sending a letter from yourself as the accountant then perhaps a letter of clearance as mentioned on another thread?
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As mentioned above, you need to send a letter of professional clearance requesting copies of last years accounts and tax computations. Your dad should also probably write a letter to them stating that he no longer requires their services.
I usually send something along these lines but it does depend on the type of client.
Dear Sir,
Re: Michelles Dad
I have recently been approached by the above named client to act as their accountant and tax adviser. Please could you inform me of any circumstances or information which you have knowledge of, which I need to consider in deciding whether or not to formally accept this appointment. If there are no such circumstances that I need to be aware of, would you please supply the following information:
1. The last tax return and schedules submitted. 2. Business taxation computation, capital allowance claims etc. 3. The last completed accounts available for the business. 4. Any schedules you have giving the details of the make-up of balances in the last completed accounts, including: (a) Debtors and prepayments (b) Creditors and accruals (c) Bank reconciliations for all accounts (d) Stock (e) Fixed asset register
5. Details of any taxation losses available and overlap not previously utilised. 6. Any other schedules etc., which
Obviously tailor the letter to suit the client, so if the client is a cis subcontractor who only ever has a simple @income & Expenditure' account, there won't be a balance sheet, probably no creditors and maybe not even a business bank account. Getting the CA's is very important though!