Annualising working hours (AH) is a means of achieving flexibility in the use of human resources to face the seasonal nature of demand. Some existing planning procedures are able to minimise costs due to overtime and temporary workers. However, due to the great difficulty of solving the problem, it is normally assumed both that holiday weeks are fixed beforehand and that workers from different categories who are able to perform a specific type of task have the same efficiency. Often reality is different, thus, there is a gap between academic and real problems. In the present paper, those constraints are relaxed and a much more general and close to reality problem is solved in an exact and very efficient way.