Ziggypop Stanley

I hold 2 unit trust/OIEC accumulation investments started in 1968 and 1977. All dividends have been reinvested and declared on my tax returns. How is the capital gains tax calculated if I sell now? The original investment certificates only state the number of units purchased not the actual amount paid. The original Provider has been taken over several times and the current providers do not have records going back to the start. My understanding is that the total value of the accumulated dividends over the last 54 and 47 years are treated as the acquisition cost for CGT purposes - is this correct?