Capital Gains on Stock Option Sales

I work for a US domiciled company and have been granted stock options. My understanding is that capital gains tax will only be applicable from the date that the stock options are exercised. Therefore, if the options are exercised and sold immediately, the full gain arising will have bene subject to income tax so no capital gains will be due. Alternatively if the shares are retained after exercise any future growth in value will be subject to capital gains tax on disposal. 1. Can you confirm that my understanding is correct? 2. If I have made a mistake in this regard on previous tax returns (i.e. I have applied CGT on the money arsing from exercise/sale in same transaction, can I apply to HMRC with documented evidence to explain to seek a refund? If yes, for how many tax years can I go back in this regard?

Hi

You are correct.  You pay income tax when the shares vest.  If you retain the share and dispose of them at a later date, you may be subject to capital gains tax on the difference.  
You have one year after the due date of your tax return to amend it (e.g. 2022 to 2023 return can be amended up to 31 January 2025).  
Once the date for amendments has passed, you will need to submit an overpayment relief claim OPR).  
There is a specific format for OPR claims, that must be followed.  Please have a look at the guidance at SACM12150 - Overpayment relief: Form of claims.

Thank you