How to calculate capital gain for stocks purchased long time ago?

Hi HMRC, I have some stocks on hand now and have some questions about the capital gain. Could you help advise? 1. As the stocks were purchased many years ago and I cannot remember or find any record for the buying price, how can I calculate the capital gain when I sell them? 2. If the stocks are in Bank A currently, and I move them to Bank B, can I use the price when they are moved to Bank B as the "buying price" to calculate the capital gain? Looking forward to your reply, many thanks.

Hi

1. You will need to contact the company that you purchased them from or ask us to arrange a post valuation check - https://www.gov.uk/government/publications/sav-post-transaction-valuation-checks-for-capital-gains-cg34
2. No you use the price on moving them to a different bank

Thank you