Missing base costs for CGT

I thought this had been discussed here before but I can't find it. What should we do if we have no base cost for CGT calcs and is there a difference for pre 1982 assets? This is useful post on HMRC forum but it doesn't go into much detail about "reasonable estimates".
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Comments
It's a classic CFP 'reasoned and reasonable' approach to the estimate. HMRC will want to see that meaningful efforts have been made. When we've done these previously it's a case of tracking back as far as possible.
I always assume that HMRC must deal with a lot of these, and probably accept the well intentioned efforts of individuals to arrive at a reasonable non-zero number, especially when an agent is acting for them (proceeds of crime etc).
For 1982, isn't that effectively the backstop date i.e. everything is revalued to that point anyway so earlier acquisitions don't matter?
Thanks @benjaminfabi That's what I thought with 1982.
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