AB5301 Bolton

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?