Annualising working hours (AH) is a mean of achieving flexibility in the use of human resources to face the seasonal nature of demand. Some of the existing planning procedures are able to minimise cost due to overtime and temporary workers but, due to the difficulty of solving the problem, it is normally assumed both that the holidays week are fixed beforehand and that workers from different categories who are able to perform specific type of task have the same efficiency. In the present paper, those assumptions are relaxed and a more general problem is solved. The computational experiment leads to the conclusion that MILP is a technique suitable to dealing with the problem.