Exact base cost not available

In 1989/90 I made monthly contributions to a unit trust and built up a modest capital sum. At the time I did not think CGT would ever be an issue with indexation allowance and subsequently taper relief. Even when these were abolished, the annual exempt amount meant CGT was unlikely to be an issue. I was confident I had sufficient information to keep within the annual exemption, so would not need to make a declaration to HMRC. Although I can get a reasonable estimate from the FT website of the base cost, I am not able to calculate the precise figure. Additionally, as they are accumulation units, the reinvested dividend payments over the last 34/35 years can be added to the base cost. However this figure does not seem to be readily available. However, with the radical reduction in the annual exemption CGT has become an issue. Ignoring the reinvested dividend income, the gain is likely to be in the region of £1,500 which would be taxed at 10% so the amount involved is quite small. (The annual exemption has been used up on the sale of small inherited plot of land.) How should I proceed when precise figures are not readily available?

Hi ,
 

"you can use reasonable estimates but you must refer to this in your computation.  If your computation includes estimated figures or a valuation, you must tell us:  • where and why you’ve used estimated amounts  •  how you worked out these amounts   For valuations, you should also include:  • the date of the valuation  •  a full description of the asset  . Thank you .