Calculating the cost of shares sold

I bought six tranches of the same listed shares over the past few years in a trade account. I sold a portion of these shares in 2023. Do I need to use the average cost for all the shares I purchased over the past few years, or can I allocate the sales to the shares I purchased based on the oldest sold first. I read about Section 104 but it seems to apply to employment share schemes, whereas my situation is just an online listed share trade.

Hi Diane Cannon,
Please have a look at the guidance on S104 holding, for advice on how to work out the value, in helpsheet HS283 here:
Shares and Capital Gains Tax (Self Assessment helpsheet HS284)
Thank you.