Capital gains on multiple demerged shares

How do I calculate the capital gains on inherited shares where the company has undergone 2 demergers and not all shares have been sold. Example; I inherited some 3000 shares in company A in 2011 (5p ord shares). Company A demerged in 2019 leaving me with 3000 5p ord shares in company A plus 200 shares in company B, there was no additional financial settlement. - Considered a reorganisation of shares no CGT was liable to be paid. Company A further demerged in 2021 leaving me with 3000 5p ord shares in company A still and a further 10 shares in company C. Again there was no additional financial settlement and this was considered a reorganisation therefore no CGT was liable to be paid on this date. I have sold the shares for company A and B and retained the shares in company C, how do I work out the gain? None of the examples in the help sheets seem applicable

Hi,

You can see guidance here:

Capital Gains Tax, share reorganisations and company takeovers (Self Assessment helpsheet HS285)

Thank you.