Damian works in the Public Sector- it is against the law for the public sector to conduct 'post tender negotiation' after a contract has been awarded. This is because it's not transparent and equal opportunities, which are founding principles of public sector procurement.
Private sector can do whatever they want. They can do post-tender negotiations.
NOTE: this is currently the rules as per Public Contract Regulations 2015. These are being superseded in October by new legislation. The study guide doesn't mention this, but it's happening. At the time of writing (January 2023) we don't have the full details about what the new rules are but I doubt they will change on this particular topic. You can find out more about the changes to Public Sector Procurement here: Transforming Public Procurement - GOV.UK (www.gov.uk)