PAYE47120 | Employer returns: employer return - overpayments: trade dispute: refund not made at due time
From HM Revenue & Customs · PAYE Manual
It may not always be possible for an employer to pay a suspended refund to an employee. For example the employee may leave without
Giving notice
Or
Leaving a forwarding address
If a refund is not paid within 42 days of the due time the employer must
Pay the sum to the Debt Management Office
Add the tax not refunded to the final figure of tax on form P14 at the end of the tax year
Show the tax due according to the tax tables where form P45 is issued in the same tax year