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I acquired shares in my employing company via SAYE for more than 20 years. Previously when I sold some shares, I sold a specific share certificate with a specific number of shares, and calculated the capital gain from my record of what I'd paid for those shares; each time I made sure that the capital gain was less than the Annual Exempt Amount for that tax year.
In February 2024 (2023/2024 tax year) I sold most of my accrued shares, to fund a house purchase. This time there is a sizeable capital gain, which I initially calculated in the same way as previously, i.e. from the gain relating to the shares on each of several share certificates.
However, I now understand that I need to calculate the capital gain using the Section 104 mechanism. The difficulty with this is that I don't know the cost of acquiring the 2675 shares I held on 5 April 2008. How do I account for this in determining the initial 'pool of actual cost', so that I can carry this forward in accounting for the additional purchases and sales since 5 April 2008?