NEC ECC: Introducing a damages free time period due to Covid-19 delays

I am working on a NEC4 Option A project, where the contract has been adjusted so that the prevention (60.1.19) clause and other related clauses have been omitted via Z Clauses (originally to do with Brexit) and there is no X2 clause. As the project has been designated a key project, the Client is keen the works continue (if the Contractor believes they can complete the works in line with all guidance), therefore the risk of Covid-19 has shifted on to the Contractor. The Contractor has reviewed their programme and to maintain the safe working believe the programme will be extended by 18 weeks.

The Contractor has requested and the Client has agreed to a Liquidated Damages free period for the length of delay caused by Covid-19. As a team, we have thought of a couple of ways this could be introduced, but I was wondering if there is a correct way? Which will maintain the contractual balance, but will give the Contractor comfort they won’t be levied damages at the end of the project.

The easiest way would just be to write a Deed of Variation which says that instead of the Completion Date being where it is currently [state date], it is put back by 18 weeks to [state new date] and that all other compensation events notified and in process i.e. not implemented remain notified and in process.