How does JCT clause 4.20 work?

Clause 4.20 of SBC/Q 2016 entitles the Contractor to reimbursement of the direct loss and/or expense it incurs because regular progress of the Works has been, or is likely to be, materially affected by a Relevant Matter, and that entitlement is subject to the notice and information requirements in clause 4.21. It is the money provision, separate from the extension of time provisions, and the two run independently. The mechanism is in clause 4.21. The Contractor notifies the Architect/Contract Administrator as soon as it has become, or should reasonably have become, apparent that regular progress has been or is likely to be affected, identifying the Relevant Matter. With the notice, or as soon as reasonably practicable afterwards, it gives its initial assessment of the loss and expense incurred and likely to be incurred, with the information reasonably necessary to allow it to be ascertained. It then updates that assessment monthly until all the information needed for the final ascertainment has been supplied. The Architect/Contract Administrator, or the Quantity Surveyor on its instruction, ascertains the amount within the periods the clause states, first for the initial assessment and then for each update, and the amount ascertained is added to the Contract Sum and picked up in the next Interim Certificate. Direct loss and expense means the cost actually caused by the Relevant Matter, prolongation of preliminaries, disruption to labour and plant and the like, built from records rather than a percentage. Clause 5.10 keeps that money out of Variation valuations so it is not paid twice. The failures are predictable. The notice is given when the cost is counted rather than when the effect became apparent, the initial assessment is a round sum with nothing behind it, the monthly updates stop, and the ascertainment is left to be argued at the final account when the clause wants it done cycle by cycle while the records are fresh.