Re: [Business Name & Individuals Names]
After careful consideration, [I/we] have decided to use the services of a different accountant to undertake all our accounting and taxation work.
[I/We] have asked Murphy Salisbury Chartered Accountants to act as [my/our] accountant and professional advisor.
[I/We] authorise you, as [my/our] former accountant to provide them with any information they require. [I/We] thank you in advance for your prompt assistance in dealing with their requests.
[I/We] would also like to take this opportunity to thank you for the assistance you have provided over the years and wish you all the best for the future.
Yours sincerely
[Your name]
[Position within company]