Old CGT registered losses. How long do they stay on HMRC files?

I have a fair number of Capital losses dating back some 20-30 years, which I registered with HMRC at the time. I have not done much share dealing since those days and therefore have not had to send in any capital gains pages with my tax return for quite a number of years. I now have begun to liquidate shares as I am now retired, and will need to use those losses to offset any capital gains on my share sales. I hope my next tax return filing will go smoothly, but is there anything I should be aware of regarding where on the return I should put the "old" losses? And will I need to re-prove/justify those loses to HMRC or will HMRC files still hold the information? Thanks in anticipation.

Hi john bowman,
As this relates to pre Self Assessment years, the information will not be on your tax returns.
You will need to cotact the Self Assessment helpine for your record to be accessed to make sure the details are held.
Self Assessment: general enquiries
Thank you.