Tax free of old

My client holds a single premium life assurance policy which he initially took out in 1967. He recently part encashed this which resulted in a significant gain. He has now asked me whether there are any consequences for income tax or capital gains tax that he needs to take into account when considering other investment decisions such as reviewing advice on further possible sales of stocks and shares to use his annual capital gains tax exemption.
The life office suggests to my client that the proceeds are tax-free but cannot give the reason to support this. Textbooks refer to the position of post-1968 life policies but not earlier ones.
Can any...