CGT on a milestone payment

A previous employer was acquired by another company and all employees shares were purchased as part of that deal about 4 years ago. I paid CGT for my shares that were purchased as part of that agreement based on the difference between the purchase price and the cost basis to me when I was granted the shares. If certain conditions were met at a later date, the acquiring company agreed to pay an additional disbursement per share to the previous shareholders - approximately £0.50 per share. I assume, therefore, that this additional payment is also subject to CGT at the appropriate rate even though I sold the shares 4 years ago.

Hi,
Please refer to:
Capital Gains Tax, share reorganisations and company takeovers (Self Assessment helpsheet HS285)
Thank you.