How do I calculate the CGT on shares Im now selling

I have been buying shares in an investment trust ( outside of an ISA) most months over the last 10+ years ( so 120 +purchases) When I want to sell some, how do I calculate the CGT? Do I need to find the paper work from when I brought the shares and calculate the difference in the shares price? When I sell, are the "the oldest", or the newest shares sold?

Hi cp3,
If the share are of the same company and same kind, you can pool them together and use an average value for the acquisition cost.  This will be deducted from the disposal value, along with broker fees etc, to work out your gain/loss.  In this way it does not matter whether the share disposed of is older or newer than any other disposed at the same time.  This is known as a section 104 holding.  
Please have a look at the guidance at helpsheet HS284 for more information. Shares and Capital Gains Tax (Self Assessment helpsheet HS284).
Thank you.

Thank you for your reply. Looking at example 3, it looks like I need to go through every purchase of shares in a company to determine the cost of the pool? ... then it gets easier because of the pooling. I can only find the contract notes from the last 7 years. If I am unable to find the contract notes, what should I do.

Hi cp3,
You would need to contact the company direct od the London Stock Exchange.
Thank you.