Hi Paul Murphy,
If you are resident in the UK you are required to declare your worldwide income in your Self Assessment tax return.
Your foreign gain would be declared on SA108
If foreign tax has been paid on your gain then you should also complete SA106
to claim your foreign tax credit relief.
Please see page 6 of SA106 at question 33 onwards – Capital Gains – Foreign Tax Credit Relief and Special Withholding Tax.
You may find the following links helpful.
Report and pay your Capital Gains Tax: If you have other capital gains to report
Private Residence Relief (Self Assessment helpsheet HS283)