Car benefit

My client has been contacted by the Inland Revenue with respect to tax years dating back to 1999.

They claim there is insufficient evidence to prove that my client drove over 18,000 miles in those years.

The company my client worked for has since gone into administration and has not kept any diaries of this time working for that company.

What is the best way to approach the Revenue in this situation

Thank you in advance
sarahkedgley