How to claim for capital gain or loss when losing all purchasing transaction records

Hi, I have various stock shares for more than 20 years and I lost all records / proofs of these purchasing orders including unit price and date of buying. I just have my own written record about when and how much the shares being bought. When I moved to UK last year and is a UK tax resident now, I need to submit self assessment tax return and report the capital gain or loss (if any) to HMRC. Could you please advise me how and what I should do if I have lost these purchasing transaction records? Thank you in advance.

Hi KuenHK Lam,
If you have your own written details of when purchased you would then be able to contact the companies concerned to confirm the price at aquisistion.
Thank you. 

Hi HMRC, Thank you for your advice. I bought and keep different stock shares via various banks over 20 years. I don't think the companies or individual banks are able to re-print the related transaction orders / records in response to my requests. That is why I ask for HMRC's further advice at this forum. I wonder if I can base on my written records to prepare the related calculation worksheets for capital gain/loss reporting. Or, I am required to provide previous purchasing transaction records for verification, otherwise, no claims for capital gain / loss can be reported to HMRC. I am looking forward to receiving your expert comment. Thank you in advance.

Hi,
Provided you attach a computation and in the absence of any other paperwork from the companies, you can base your calculations on your own written records.  
Obviously, we can enquire into those calculations and your figures under the normal enquiry rules.  
Please have a look at:- Capital Gains Manual CG16600P - Capital Gains manual: introduction and computation: Valuation of assets for Capital Gains Tax: valuation procedures: post transaction valuation checks, if you wish, you can apply for post transaction valuation checks using the CG34. Post transaction valuation checks for Capital Gains (CG34)
Thank you.