Capital Gains on an asset acquired over time

My company gives me restricted stock units of our publicly listed shares every quarter.
This financial year I started selling them. The first sale I made covered units acquired over 1.5 years and due to bed and breakfast rule it also includes units I was given by the company 15 days after the sale.
In the self assessment forms, I am required to declare each disposal.
However, it also asks me to declare the date and the cost at which I acquired the asset.
Given that for this disposal, the asset was acquired at different dates, what should I enter in the form?
Thank you.

Hi Vector Boson
Please see the guidance at: 
Shares and Capital Gains Tax (Self Assessment helpsheet HS284) and in particular the S104 holding example
Thank you.