Below is a clear guidance that has just been updated by HMRC for circumstances where a business is re-imbursing or assuming an expense that has been paid from an employee (including directors) personal money.
“When an employer reimburses non-deductible expenses to an employee, who has paid the initial cost themselves, the reimbursed amount is treated as earnings which is subject to PAYE deductions.”

https://www.gov.uk/government/publications/cwg2-further-guide-to-paye-and-national-insurance-contributions/tax-and-primary-class-1-national-insurance-contributions-on-expenses-and-benefits-paid-by-employer