How long do I need to keep evidence

In Tax Year 2019-20 I made under £7,000 capital gains. Then in Tax Year 2020-21, I made under £9,000 capital gains. I checked the guidance at the time and because it was under the Capital Gains Tax allowance I did not need to file a return. However, how long do I have to keep evidence for the transactions. I have looked everywhere and cannot find the answer. All I can find is that if you file a return you have to keep for at least 12 months. However, I have never filed a self asse return as I have never exceeded the allowance. Please could you tell me how long I have to keep the evidence. Also, please make your guidance better. I have spent the past 2 days looking for the answer but cannot find it. There are millions of people who would be in my situation. It should not need you to be Sherlock's Holmes to find such a answer to such a straight forward question.

Hi ,
 

The tax year 19/20 is now out of date to report these capital losses for them to be used at a future date. for 20/21, you should send in a letter with a computation to show how the losses arose. These can be noted so that they can then be used whenever you need them . Thank you .