How does NEC4 clause 53 work?

Clause 53 is the NEC4 final account, and its distinctive feature is a short, hard deadline after which the figure is conclusive. Under clause 53.1 the Project Manager makes an assessment of the final amount due and certifies a final payment, if one is due, within four weeks of the Supervisor issuing the Defects Certificate, or thirteen weeks after the Project Manager issues a termination certificate. The assessment uses the same building blocks as every interim one: the final Price for Work Done to Date, the other amounts payable in each direction, and, under Options C and D, the final assessment of the Contractor's share under clause 54.4. Under clause 53.2, if the Project Manager does not make the assessment in that time, the Contractor may issue its own assessment of the final amount due, and the same consequences attach to it. Under clause 53.3 an assessment of the final amount due is conclusive evidence of it unless a Party refers a dispute about it to the dispute resolution procedure in the contract within four weeks of the assessment. Clause 53.4 records what happens when the dispute procedure changes the figure: the assessment is revised to include the outcome. In practice the final assessment is the last place where unimplemented compensation events, disputed Disallowed Cost and the share are picked up, so it must be prepared from a register of every open item rather than from the last interim certificate. What goes wrong: Contractors wait for the Project Manager and do not use clause 53.2 when the four weeks pass; nobody diaries the four weeks in clause 53.3, and a figure both sides were still negotiating becomes conclusive; a Contractor's own assessment is issued without the records to support it; and the parties assume that continuing to talk suspends the period, which it does not.