To work out the capital gain it is necessary to know which units Alan sold and how much they cost.
Special 'share pooling' rules exist to deal with this problem.
Shares of the same class in the same company acquired at any time by a person in the same capacity will normally become part of the 'Section 104 holding'. The Section 104 holding is simply the share pool. However, shares that are identified with acquisitions under the 'same day' or 'bed and breakfasting' identification rules do not become part of the pool.
The Section 104 holding is a pool of qualifying expenditure as regards the number of shares in the holding.
The pool grows whenever further shares are acquired that enter the pool and reduces when there is a disposal of shares from the pool (shares identified in accordance with the first two bullet points below do not enter the S104 holding).
Legislation provides that disposals must be identified in the following order: