Before chasing this, you need to get the facts straight.

There is no obligation for the company to declare p/mile reimbursement for business travel in private cars. The Government operates the Approved Mileage Rates scheme allowing 40p/mile for up to 10,000 business miles per annum, and 25p/mile thereafter. Amounts above these levels are the employees responsibility to declare for tax. Below that level, the individual can claim tax relief on the shortfall. All the firm should be doing is getting the driver to complete some kind of expense claim for their book-keeper to record the payment as a business expense and minimise the profit (and tax) payable by the company.

Directors are the same as any other employee and should pay Benefit In Kind tax on any car allocated to them and available for private use. How do you know they aren't? (The form is a P11D, by the way)

But given what your ex-director has said, I don't think I'd risk a kicking for £50! If you have proof of the Directors evading BiK, I'd make an anonymous call to HM Revenue and Customs and enjoy the satisfaction that the expense of complying with an investigation will be way more than the £50 would have cost them!!!