Hi Charles David Hume,
As the payment is from your employer, the income should be shown in the employment section if it is included in your P60.
You would then claim credit for the Tax in the foreign section under 'Employment, self-employment and other income which you paid foreign tax on'.
If it's not included in your P60, please include it on the box on the employment page for 'Tips and other payments not included on your P60'.
ERSM20193 advises that when RSUs payout at the market value on what is called ""dividend equivalents"" in either cash or shares, such payments will generally be taxed as earnings in the year they are received. https://www.gov.uk/hmrc-internal-manuals/employment-related-securities/ersm20193
For Capital Gains - sell the shares immediately upon vesting. This ensures that there is no gain to tax. If you still want to hold the shares, you could buy them back in a stocks and shares ISA. This ensures that any future growth is tax-free (although you may still pay withholding taxes, particularly if the shares are held in a US company).
If you hold the shares within a SIPP, any future growth is tax-free and no withholding tax will apply (assuming that your pension administrator has set it up correctly!).