If a contractor puts a high rate for an item under JCT Standard Building Contract 2009 for which it was thought only a small number would be required, and ultimately far more of the item were required, is a quantity surveyor entitled to reduce the unit ra

Generally, no. There is some provision for the quantity surveyor to reduce the unit rate by a small percentage to reflect the economy of scale, but in general terms the contractual unit rate must be honoured.