payment for labour required for variations or day works shall be at the rates nominated in this schedule. the rate shall be firm and not subject to rise or fall and shall be deemed to include the cost of wages, contractor's profit, overhead, superintendence, insurance, timekeeping and all clerical and office work, professional and technical service, royalties,taxes,as well as transport about the site, and ues of all hand operated tools or plant and all incidental charges whatever.