RSU - Witheheld tax

I have a client who has with held money from employees for vesting RSU's through payroll through a post tax deduction as advised by there accountant to pay the tax. This tax now needs to be paid across to HMRC, where does this get paid to and what reference numbers should be used so it is allocated correctly? Thanks,

Hi
They should pay the tax along with their normal liabilities using the account office reference.
Thankyou.