Quick answer

Yes: an agreed contract rate governs the work, even where the quantity increases substantially and even where the rate was a mistake, so the low rate applies to the increased quantity, as Henry Boot v Alstom shows, though a quantity surveyor who spots an error on vetting should point it out

Yes. An agreed contract rate governs the work, even where the quantity increases substantially and even where the rate was a mistake. Henry Boot Construction Ltd v Alstom Combined Cycles Ltd went the same way, the Court of Appeal holding that a mistaken rate still governed a variation, because the contractual requirement of reasonableness went to the process of applying the rate, not to the rate itself, so the parties were stuck with the rate they had agreed. There are two qualifications: a quantity surveyor or engineer who spots an error while vetting a priced bill should draw it to the contractor's attention, or the contractor may become entitled to an adjustment; and the rule addresses errors in the rate, not a change in the nature of the work, so in Aldi Stores Ltd v Galliford moving material from one category to another, where both rates were nil, gave no variation.