Using the simulation models a methodology was developed for leakage detection and localisation in a water distribution network [4]. Such methodology is based on fault detection and isolation techniques. Sensitivity matrix of pressures to leaks [8] is generated using simulation models [1]. Such models are available in water companies. The methodology showed promising results but two main issues appeared. Calibration in models and precision in sensors was required. Investment in sensors could be afforded [6] as long as models were well calibrated enough. The main uncertainty in models lies on demands. This paper presents a study of the effect of an improvement in demand calibration on isolability of leaks. The aim is to quantify the return of an effort in calibrating in terms of leak detection one of the main issues in water industry [2].