Tax on sale of shares issued on demutualisation

I received shares on the demutualisation of Norwich Union (now Aviva) and Standard Life (now Aberdeen Standard) as I was a member by virtue of having pension schemes with each. What is the tax base cost I should assume when calculating any gain on the sale of this shares as I haven't bought these shares or any others since.

Hi,
You will need to check the value at the date you acquired them by checking with the London Stock Exchange.
Thank you.

This reply seems to suggest that the base cost for assessing a gain for cgt is the market price on demutualization. However, the following resources suggest the tax base is zero... https://www.gov.uk/hmrc-internal-manuals/capital-gains-manual/cg56820 https://www.legislation.gov.uk/ukpga/1992/12/section/217 Please could this be clarified.