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.