Some funds wrap dividend payments or fixed interest payments back into the fund price. You get a certificate each year of how much these payments were, and they count as income in that year's personal tax assessment.
So when you finally dispose of the fund, or a portion of the fund, you can't just take the selling value and subtract the original buying value, because some of that value increase has already been taxed as annual income. Presumably you have to subtract the accumulated income from the increase in value in order to arrive at the capital gain. If you've held the fund for many years this could be quite an exercise...
So are there any tools that extract the capital gain from an ISIN given the purchase date and the sale date? Such a tool would have to examine market feeds to extract rolled-up income information. It sounds like a perfectly plausible thing to exist as it ought to greatly reduce the potential for error and it feels as if it would be a common requirement for personal tax calculation where individuals have invested in funds.
Maybe I'm completely misunderstanding how capital gains on ISIN's with rolled-up income work, in which case please be gentle!