A Contractor undertakes work which includes erecting blockwork to form ducts for mechanical services described as isolated casings in the Contract Bills. He programmes his work so that this element can be carried out at the same time as his general blockwork. In the event, instructions related to the mechanical services are delayed and, as a result, the enclosing blockwork has also to be delayed and thus executed out of sequence and in a piecemeal fashion. The failure of the Contractor properly to manage the works resulting in a loss of bonus will not of course qualify, nor would the failure to achieve unduly optimistic productivity implicit in any bonus target. Where the regular is constantly being interrupted, the Contractor might claim that any attempt at running a bonus scheme had to be abandoned and some form of average weekly payment substituted in lieu became necessary in order to attract men to the site.