CGT on US Stocks

I am a UK taxpayer working for a US company in the UK. As part of my package I receive stock awards which vest on a regular basis. The amount I am awarded has an amount withheld at source to cover the US tax withholding requirements. For example, if I am awarded 100 stocks then I only actually receive 70 in my brokerage account (fictitious numbers!).


I am now thinking of selling some of the vested stocks but I'm not sure if I will then be liable for CGT.
How would the potential gain be calculated? Is it the sale price minus the price of the stock when it was awarded or when it vested?
The vesting goes back over years, so would I have to find the price of each batch?


Or does the fact that a number of stocks were withheld when originally awarded mean that this is already covered?


Thanks for any advice!