Reporting disposal of foreign investment even though no funds released yet?

Hello, I'm a UK citizen and basic rate tax payer waiting on foreign investment information to enable me to complete my 23/24 self assessment. In March 2024 I initiated the sale of my South African unit trusts but to date have not received the nett funds due to South African Revenue Service and Exchange Control red tape. I do have the gross figure (in Rands) from the sale of the unit trusts (approx. £10k equivalent). I expect there will be a South African non-residents capital gains element to be deducted at source (as historically through the last 34 years of this investment both South African and UK tax has been paid on interest and dividends declared as per the double taxation agreement). It is now unlikely I will have the nett financial info in time to include this on my self assessment. My question is: 1) Can this March 2024 disposal and subsequent CGT payment be declared to HMRC in the future and not in the self assessment I'm currently working on?

Hi
You will need to provide estimated figures in the 2023/24 tax return for the filing deadline and you can then amend it once you have the actual figures.

Thanks for this advice. Once I have actual figures how do I inform HMRC? Simply write a letter containing the information or is there a specific form or other mechanism?