Private US Company Restricted Shares Grant

I am maybe going to receive a grant of RSU from a US company- which is private with no current evaluation (pre-seed). It is not registered in the UK. When - at what point - do I have to pay capital gains tax- as the company has no valuation and no market price?

Hi,
When the RSU's vest, your employer will use some of the shares to cover the income tax that arises.
 If the shares are disposed of immediately on vesting, there is no capital gains tax.  
This is declared in the employment section of the self assessment tax return and also in the foreign section of the tax return, so that you can claim a foreign tax credit.
If you hold on to them for a period of time after they vest, then they may also be subject to capital gains tax on their disposal.
Thankyou