Calculating CGT on OEIC disposals where there is a regular monthly investment

I have an OEIC which was set up with a small lump sum in 2000/01 and subsequently I have been paying in £50 per month for the last 24 years. The fund has built up over that period, less a small withdrawal and when I buy new shares the price varies from month to month. It is an accumulation fund where the accumulation increases the value of the shares rather than paid out. If I dispose of all or some of the shares, how do I calculate the cost of purchase for CGT purposes? I have read I can calculate an average share paid amount from the total amount paid for shares divided by the number of shares held now and compare that to the price per share on sale date. Can I include the accumulation figures in the amounts I have paid in to the OEIC as well as any commission costs paid before calculating the gain?

Hi,
Please have a look at CG41562 onwards for guidance in this area:
CG41562 - Open-ended investment companies
Thank you