Calculating CG on shares without purchase information.

Hi, About 30 years ago I was given an option certificate from my employer, to purchase shares at a discounted rate if they ever went public. I had to hold the certificate for a fixed number of years after receiving it before being able to use it. About a year before that time was up, they went public, then when the date came around I bought the shares, With the help of an accountant sold enough to max out my capital gains allowance for that year. The following year, I sold the maximum allowance again. The certificate for the remaining shares was lost in time across at least three house moves. Since then, around a decade ago, there was a "Return of value and listing of new ordinary shares", which reduced the number of shares but increased the value of each one. At that point, the company sorted out Equinity online access and I can see the "new" shares allotted from the corporate action plus a cash payment of £165.19 (and tax paid of £18.35). I have found the original Regulatory news and it looks like for every 100 old shares I got 88 new shares. As I do not have accurate records for the who thing start to finish, what is the accepted method to calculate capital gains if I sell some of these remaining shares? Do I assume 100% is profit as I cannot prove otherwise? Thanks.

Hi Roy_F,
Please have a look at the guidance at the start of the share valuation form.
If you circumstance are covered in this guidance, you can fill in the form and request a value.
Request for a share valuation.
You can also contact the shares and asset valuations team here:
Shares and assets valuations.
Thank you.