The basic position is that PAYE must be operated on post-acquisition charges if:
•the employment-related securities are RCAs before the chargeable event that gives rise to taxable employment income; or
•the consideration received by the employee as a result of the chargeable event takes the form of cash or RCAs.
In practical terms, this means that tax charges arising on shares in listed companies will always be subject to PAYE and NIC.
Assuming that private company shares are not RCAs, then tax charges arising before the final disposal of the shares will be self-assessment matters, while charges arising on disposal will be subject to PAYE and NIC.