Your first port of call would be to establish with the employer or wages department the breakdown of the award received. As we understand it the RSU’s can be paid out as cash or as shares. Any sum paid out as cash is included in pay, and taxed accordingly.
However, there may still be a deemed benefit in kind for the shares awarded, due to the difference between the sum you are deemed to have been awarded them at and the market value at the time. You can see guidance and examples here:
ERSM20193 - Employment-related securities and options: what are securities: RSUs and dividend equivalents
ERSM20194 - Employment-related securities and options: what are securities: RSUs and dividend equivalents: examples
If there is any error in the P11D or Real Time Information information provided by the employer they will need to send revised forms and make any Earlier Year Updates necessary to your records. HMRC can then reconcile the year and refund any overpayment.
Failing this, you would need to provide all necessary information in connection with the RSU and sums awarded with any cash payment details for HMRC to review.
It maybe that the options have been granted under the law applying to the USA, in which case HMRC would need to seek advise from the Employee Shares and Securities Unit (ESSU). You can see more information here:
ERSM10040 - Introduction