CGT on partially sold fund

I have investments in a few ETF funds, and just partially sold one of these. The complication is I've been investing weekly/monthly so there isn't a consistent buy price and I still have some of the investment left, so how do I work out the value for CGT? Are the oldest investments always sold first for instance, or the newest? I know the gain is less than the limit for the year, but if I sell more, I need to know how much of my allowance I have left to try and avoid tipping over the allowance. Many thanks, 

Name removed admin .

Hi,
You would treat this as a S104 holding. see guidance at Shares and Capital Gains Tax (Self Assessment helpsheet HS284)- select both parts for a full review.
Thank you.